Improved Github support.

[cptwunderlich]

Filtering out 'draft' PRs.
Checking PR's 'mergeable' flag as merge-check.

[cptwunderlich]

I'm filtering out 'draft' PRs to begin with, bc. neither can nor do we want to merge them (unfortunately, the API doesn't allow us to filter them with the request).

The second thing is about the merge check.
As the docs state, Github will perform a merge-check in the background (and that can go stale). So we request the PR item resource, which triggers a merge-check (if none or stale). While that is running, the value of "mergeable" is NULL.
My retry mechanism is still a bit sketchy. We'd actually have to repeat the request (with a little waiting time) until we get a Non-Null result.

[cptwunderlich]

I improved the retry mechanism. So far, delay time and retries are hard-coded though.