update it all to actually use .NET45
include the #ifdef for NET_45
dropped a bunch of async/await usages

There's also some differences here between using IReadOnlyCollection and
IReadOnlyList. We should decide on one and be consistent
Merge pull request #117 from octokit/async-await-is-addictive-like-cocaine

It really should be .NET45 - and less async/await