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 actually created to fix a bug (#128). Thus I would think the following makes more sense:

```
As a developer refactoring my branches
I should receive a useful error message when git-extract errors because the branch name is taken
```
I don't think the so would be necessary in this case.