git kill branchname

@charlierudolph Aaaaahhhh, for some reason some commits meant for this PR ended up in an already merged PR. I was messing around with `git extract` while being distracted on the resource planning call, so that's my (super lame) excuse... Anyways, please take a look at https://github.com/Originate/git-town/commit/738e3e76158a4803c5695abf71349b21d02033f5, and comment there. I will implement the corresponding fixes in this pull request, and once this gets merged its all going to be kind of okay again...  :smile: 
The `on_branch` method should only do what its name says it does: do something on the given branch. If we deviate from this basic principle, it will become quite hard to understand what is actually going on in the code. Stashing away open changes is a significant side effect, and the fact that it happens should be apparent to anybody who uses that method. I feel quite strongly about this, because I have been burned too many times by code bases becoming messed up in this way, and I have the scars to prove it.  :)

Having said that, the "verify_commits" method no longer changes branches, so I am removing `without_open_changes` here, and will reintroduce it on demand in #176 at the right place.