Add line decorators

Simple tweak now that the Gutter API landed!

Screenshot:
![image](https://cloud.githubusercontent.com/assets/253202/3225392/a4b2f92a-f04f-11e3-8ab3-51cfc999641a.png)


It appears `isSoftWrapped` is a function [src/tokenized-line.coffee#L103](https://github.com/atom/atom/blob/master/src/tokenized-line.coffee#L103).

Writing tests for arbitrary line decorations.