Stop Atom from placing your cursor past the EOL

When you click in the gutter to the right of your text, Atom currently places your cursor one column to the right of the EOL. This pull request fixes that behavior.

I wasn't sure if you all would like this approach yet, so I haven't yet written specs. I'll write specs for this as soon as we discuss it.

Before:
![before](https://cloud.githubusercontent.com/assets/4146/3140482/19d45586-e921-11e3-828a-30c7dba8fe82.gif)

After:
![after](https://cloud.githubusercontent.com/assets/4146/3140483/1f2300b4-e921-11e3-876b-0a5c82d8d039.gif)

I kind of doubt this is the right trigger; I'll bet there's something like `core:mouse-clicked` or something. @nathansobo?