Create a directive to convert a pull-request into a CL

[cgruber]

This is being done internally, but wanted to track the feature externally.

It should look something of the form:

moe github_pull --pull_request http://github.com/google/guava/pull/5 --config /path/to/config

The directive should validate that that one of the repositories is a github repository that matches the pull request, and it will attempt to stage the commits on that branch into a client/workspace in the other repository.

This might be more generalizable to something of the form:

moe stage_branch --branch repo_name:foo --root <commit> (if needed) --config /path/to/config

Really a github pull request is just fluff around a branch. But github is by far the more common use-case, so supporting that is the priority.

[cgruber]

So - this sort of exists. github_pull and merge_branch both exists. I need to sync out the docs, and update things, and there's a rather stark bug in github_pull that relates to the Migrator's logic being way too linear. I'm working on that. For now I'll leave this open.