fix notify_team on wall

Update installation docs

Closes #2213

Redesign for navigation bar.

Allow proper use of @username in Git(lab|hub) Flavored Markdown.

Allow using a custom Gravatar URL.

Default is still set to the Gravatar.com service, but allows override
for a custom gravatar-ish service.

Renamed from gitlab:app:info
Add several extra info points

always use proper size value
do not show alt text when avatar image not loaded

fix failing spec

Spelling fix to rake info task

add rake task to gather system information

Fixes #1860

Closes #2196

* build-essential requires make, gcc
* remove libmysql++-dev: the mysql2 gem only requires libmysqlclient-dev
* remove postgresql-server-dev-9.1: the pg gem only requires libpq-dev
* replace python-dev, python-pip with python2.7: the pygments.rb gem only requires python2.7