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.

まず Push と PushOptions のエントリポイントを FetchOptions と比較し、続いて既存の OnPushStatusError、OnPushTransferProgress、OnPackBuilderProgress コールバックを追跡します。完了とは、呼び出し側が push 中の OnProgress コールバックを、fetch 中とほぼ同じ方法で処理できることです。