Support for server push options

[pablonardone]

Please add support for server push options parameter. This parameter is equivalent to the value set in the git command line "--push-options"

Currentley the C library libgit2 already support server push options, and actually the internal class GitPushOptions also support it.
The problem is that this functionality is not exported by the public class PushOptions.

This functionality is extremely important so I will summit a pull request with the changes.
Thanks and I hope that pull request could be integrated as soon as possible.

[pablonardone]

Also related to the issue #2045