Only push current branch, Add debug logging

Hi vojtajina -

Thanks for created grunt-bump, we use it often and really like it.

The following pull requests fixes an edge case during the push step.  Consider the scenario where you have multiple tracked branches locally.  On the push step, grunt-bump is currently pushing all branches with a ```git push upstream``` rather than only pushing the current branch.  The main issue arises when a branch is behind its remote counterpart and the push is rejected.  In my case, the push to my master branch went through okay, but the build failed due to the err which caused my release to stop half way through.  To fix this I am using ```git rev-parse --abbrev-ref HEAD``` to get the current branch, and using that in the push (i.e ```git push <opts.pushTo> <currentBranch>```).

In addition, i added some debug logging for all the git commands which was very useful for me to see and also logged stdout as well as stderr whenever there is an err.  This is because the git error do not some through stderr.

Thanks,
Bryan