Deploy changes

Fixed a few deploy bugs; added a few deploy features, notably:

* Make branches configurable
* Use 'production' streambed branch by default on prod, but allow override to 'master' (all configurable)
* Allow locking and unlocking deployment.

@bpostlethwaite (and please tag anyone else who's been involved with plotbot).
Yeah, at some point we should save all of Plotbot's state.  Probably writing it to a JSON file is the easiest.