Use "mergeable_state" field from github api

Also, close #41
Fetch PRs with unknown mergeable_state
Check for updated mergeable status for open PRs

Fixed 42
Merged and mergeable are not filled in lists from the api !
Enhance change-events of mergeable and mergeable state
Correct bugs with RENDERER_IGNORE_FIELDS
Force the fetch of an issue following a hook

By debug i found that calling fetch_pr resulted in a 304 as we passed
the updated_at.
FetchIssueByNumber takes a new "force_fetch" parameter that will only
force the fetch to issue/pr, not "related" calls
Merge pull request #48 from twidi/check-mergeable-status

Check mergeable status