Git annex disabled by default

cc @sytse

See merge request !57

Git annex support

- [x] fix auth for git-annex
- [x] enable git-annex for repository on first annex call
- [x] config option to disable it
- [x] write tests

See merge request !55

Conflicts:
	lib/gitlab_shell.rb

Log full Net::HTTP error.

See merge request !56

Show nice error message when internal API is unreachable.

Closes #27.

Error message reads "Failed to authorize your Git request: internal API unreachable".

See merge request !54

Print broadcast message if one is available

See gitlab/gitlab-ee#241.

Uses gitlab/gitlabhq!1486.

Text is centered automatically:

```
================================================================================

                Maintenance window planned from 7AM to 9AM PST.

================================================================================
```

Longer text is wrapped automatically:

```
================================================================================

       Lorem ipsum dolor sit amet, consectetur adipiscing elit. Maecenas
    consectetur mi ac risus dapibus, sed rutrum eros elementum. Nunc id elit
     justo. Duis sagittis, orci quis fringilla tempus, odio ipsum convallis
   mauris, in porta quam nisi ac ante. Class aptent taciti sociosqu ad litora
   torquent per conubia nostra, per inceptos himenaeos. Pellentesque habitant
  morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed
   sit amet blandit diam. Quisque euismod dolor in ligula euismod aliquet in
    non libero. Aenean gravida ac augue vel laoreet. Phasellus vitae dictum
  tellus. In euismod dui sed odio tempus, eget lacinia ante fermentum. Nullam
   lacinia bibendum sollicitudin. Aenean et consequat felis. Curabitur libero
  orci, varius ac ultrices laoreet, blandit eu neque. Nullam eget semper nunc,
        id porta lorem. Phasellus at bibendum lorem, vitae sodales arcu.

================================================================================
```

Existing linebreaks are respected:

```
================================================================================

                          Planned maintenance windows:

                              February 8 7AM - 9AM
                              February 9 7AM - 8AM

================================================================================
```

See merge request !53

Create hooks response

Related to https://dev.gitlab.org/gitlab/gitlabhq/issues/1900

See merge request !52

Github importer

See merge request !51

typo in message: "accesible" -> "accessible"

Update changelog

See merge request !50

Fix custom hook output

Add missing 'require_relative' in gitlab_net.rb

Prevent 'uninitialized constant GitlabNet::GitAccessStatus'.

See merge request !49

Without this 'require' statement, you get the following error:

```
$ git ls-remote origin
/home/git/gitlab-shell/lib/gitlab_net.rb:30:in `check_access':
uninitialized constant GitlabNet::GitAccessStatus (NameError)
  from /home/git/gitlab-shell/lib/gitlab_shell.rb:63:in `validate_access'
  from /home/git/gitlab-shell/lib/gitlab_shell.rb:24:in `exec'
  from /home/git/gitlab-shell/bin/gitlab-shell:16:in `<main>'
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
```

Better git hook messages

DZ already merged it but we had to revert it because of lack of time to deploy to dev.

See merge request !48