Error when creating a new ssh key during credential handshake.

This was brought up in a [stack overflow question](http://stackoverflow.com/questions/27564755/how-to-clone-git-repository-with-nodegit-using-ssh). The code is correct and should work but the promise that is returned from the `cred.sshKeyNew` call isn't being fulfilled OR rejected.

Some investigation needs to go into getting this to work.
