Remove Rice, Release GVL

Hi,

I removed Rice as a development dependency and added code to release the GVL around `diff_main`.  I need to do many expensive diffs, so I'd like to do them in parallel.

Also, I got rid of the methods that mutate arrays. I'd rather just get a new array back than mutate the parameters.  I hope this is OK!

Here I am processing four diffs in parallel:

![parallel diff](https://dl.dropbox.com/s/i8ybowten44c4mi/Activity_Monitor_20131006_193107.jpg)

Thanks!