.gitignore updates

Some small updates to the project's .gitignore to make my development cycle easier...

* Removing .ruby-version and .ruby-gemset from repo as they are in
   the .gitignore file already.
* Adding IDE / Editor specific entries to the .gitignore for Sublime,
   TextMate, and RubyMine.

Let me know if you have any thoughts. I know this stuff is small, but it's getting annoying to stash and unstash my .ruby-gemset file, and you already had it in the .gitignore. We should either remove it from the git repo (which I did here), or remove it from the .gitignore.

Also you mentioned the `edge` branch... should I be working from that for the purposes of pull requests?

Daniel