Add ability to config personal Rotten Tomatoes API key

The hard-coded API key for Rotten Tomatoes is taking a beating. Nearly every day it is reaching either the query limit per day (10,000 queries) or query per second limit (5 QPS). I'm submitting a request to get these limits raised (apparently they handle it on a case-by-case basis), but it will still be nice for users to use their own key if they wish.

Here is the report from Rotten Tomatoes:
![rt_api_key](https://f.cloud.github.com/assets/1368827/1879815/778c50b2-794f-11e3-9b41-61c09a92496e.png)