Add comparing commits tags branches

[hknielsen]

Issue:
#432

I have added the compare and tests. Ive made the model out from the json.

Uses:
http://developer.github.com/v3/repos/commits/#compare-two-commits

[haacked]

This attribute requires that you actually have a private (or internal) property named DebuggerDisplay that returns a string. We usually only show a couple property values in such a property. The ones you'd want most to see in a debugger.

[haacked]

Thanks so much for submitting a pull request! I really appreciate it. There's a few comments I made on it.

[hknielsen]

no problem, nice work :) ive updated from the comments,pushed it too fast without reviewing it myself :D so thanks for the the comments

[hknielsen]

Hmm somehow the projects:
Octokit-Monotouch
Octokit-MonoAndroid

Fails for me, I cant open them in VS, but then I wonder why all the projects need them added, why not have a model assembly so they automatically added?

[haacked]

Run build.cmd FixProjects and commit changes.

[hknielsen]

Done :) thanks awesome!

[shiftkey]

There's definitely some overlap between this and #428

[hknielsen]

@shiftkey @haacked Yes perfect, good that it isnt a complex implementation :) If you just change your model ill close my pull request

[hknielsen]

Closing pull request cause duplicate in logic from #428