[WIP] #373 added more specific exception when PR is not mergeable

Marked as WIP. Re #373.

So to shed a little light: I sent this PR (somewhat hurriedly) during my talk this evening (http://www.meetup.com/Zurich-Developers-NET-User-Group/events/166898202/) as a demo of OSS contribution. Still a few things outstanding:

- [ ] tests
- [ ] reconsideration of the design of `PullRequestNotMergeableException` (I simply copied `PrivateRepositoryQuotaExceededException` and just made the minimum alterations to get it compiling)
- [ ] inclusion of the new exception in the Mono, NetCore45 and Portable projects (skipped to save time)
- [ ] XML docs for `PullRequestNotMergeableException` (I just ripped out the existing ones since they didn't make sense on the new type)

Have I taken the correct approach in `PullRequestsClient.Merge()`?