Also reduced a bit of duplication and
created a user.json schema file which
is referenced by pull_request.json and
pull_requests.json references
pull_request.json now.
Moved content of "url" field to "html_url"

According to https://developer.github.com/v3/users the "url" field
should contain an api url - not the html url.

Additionally Jira Pull Requests are now showing an avatar of the PR
author.

Add changelog entry

Add missing mr number

Add specs

Expose url of user in url and html_url

I decided against returning an empty string for url (which is,
according to api documentation, supposed to return an api endpoint
instead of a user profile page) but make it return the profile page
as before so the changes won't break any existing code.

Remove extra blank line