"Save" binder work as pull request back to github repostory

[mrocklin]

There are several cases in which we want to save results of work done in binder.

1. I make a small change to a notebook and want to save the result back in the home directory
2. I extend someone else's notebook and want to save the results into my own fork

Handling user accounts and github authentication is probably beyond scope. However, it might be reasonable for binder to maintain an artificial github account that submits work to repositories as a pull request that the users can then merge using their standard github account.

Thoughts?