Support OnProgress in Push / PushOptions

Feature Request:

When pushing to a remote repository its currently possible to handle callbacks for OnPushStatusError OnPushTransferProgress and OnPackBuilderProgress but there is currently no way to handle the OnProgress callback similar to the one that is available for Fetch (through FetchOptions).

This feature request comes out of this question posted on StackOverflow:
http://stackoverflow.com/questions/22149891/remote-response-when-pushing-with-libgit2sharp

I've been looking through the code to get an idea of how this should be implemented, and I'll post back with a pull request and/or additional questions once I get into it.

Commencez par comparer les points d’entrée Push et PushOptions avec FetchOptions, puis suivez les callbacks existants OnPushStatusError, OnPushTransferProgress et OnPackBuilderProgress. Le travail est terminé lorsque les appelants peuvent gérer un callback OnProgress lors d’un push à peu près de la même manière que lors d’un fetch.