Correct ajax response load on labels editor
Move prepare_fetch_headers from utils to ghpool

(and MinDateRaised from utils to models)
Remove unused merge_sha field

It's the merge_commit_sha from the api which is deprecated
Display the head_ref_restored issue event
Issue events are sorted by github_id if same created_at
Save PRs even if repo has issues disabled

We don't use the issues.github_id field as an identifier, but
repo+number because issues and PRs don't share the same id on the github
side, and in repositories with issues disabled, we don't have any issue
at all, only PRs.
Note that we have a problem with events, has issues events cannot be
fetched when issues are disabled so we don't have events for the pull
requests... i send a mail to github about this.

Close #12
Use a default user+avatar if a user is deleted from github

The username of the user is "user.deleted". Cannot be a github user
because dots are not allowed in usernames on their side
Allow fetching future closed_by if issue is reopened
Individually fetch updated pull requests
Add some pr-related fields

- mergeable
- merged
- nb_commits
- nb_additions
- nb_deletions
- nb_changed_files

+ index some fields
Call fetch_updated_prs when fetching issues
Correct delaying next issues/prs to fetch
Display "nb*" changes of pull requests

- nb_commits
- nb_additions
- nb_deletions
- nb_changed_files
Display PRs merge informations
Parse PRs links in text, not only simple issues
Retry API calls when a 502 occurs
In github.py save the ApiError code
Fetch pull requests commits
Display list of commits on pull-requests
Display "referenced" events (if commit_id)