Fix test_320_push and add methods for working with tags

On Windows Server 2008 R2, git version 1.8.3.msysgit.0
`test_320_push` all the time falls with folowing exception:

    gitapi.GitException: Error running git push ../test-clone-bare:
    Err: fatal: You are pushing to remote '../test-clone-bare', which is not the upstream of your current branch 'master', without telling me what to push to update which remote branch.

I fixed this and also added two methods for working with tags.