Describe business value of features in specs

@charlierudolph This is a preview of what I'm planning to do with all specs. Wanted to get your feedback before I invest a lot of time into this.

This helps understand WHY the feature exists. The steps then explain HOW the feature works.
This also helps formulate the value of Git Town to teams better.
This test was also inspired by a bug (#29). 
```
As a developer who wants to start hacking on a new branch
I want to be told when the branch name is taken
```