added rottentomatoes.com onebox

Hey All,

I've added a onebox for rottentomatoes.com.  

### Screens

![Screen Shot 2013-04-28 at 4 06 02 PM](https://f.cloud.github.com/assets/860486/436325/25e4d948-b03f-11e2-8f6f-d6c37f6180bd.png)


### Comments/concerns:

*  I had to override the user agent for this onebox because rottentomatoes has an advertisement for their mobile app if you visit the site on a mobile device.
*  Any recommendations about the placement of the CSS?
*  I get some warnings, and I'm not sure how to resolve them  (the first two appear on master as well):

```
/vagrant/lib/oneboxer.rb:13: warning: already initialized constant Oneboxer::Result
/vagrant/lib/oneboxer.rb:13: warning: previous definition of Result was here
/vagrant/lib/oneboxer/rottentomatoes_onebox.rb:6: warning: already initialized constant Oneboxer::RottentomatoesOnebox::SYNOPSIS_MAX_TEXT
/vagrant/lib/oneboxer/rottentomatoes_onebox.rb:6: warning: previous definition of SYNOPSIS_MAX_TEXT was here
/vagrant/lib/oneboxer/rottentomatoes_onebox.rb:7: warning: already initialized constant Oneboxer::RottentomatoesOnebox::ROTTEN_IMG
/vagrant/lib/oneboxer/rottentomatoes_onebox.rb:7: warning: previous definition of ROTTEN_IMG was here
/vagrant/lib/oneboxer/rottentomatoes_onebox.rb:8: warning: already initialized constant Oneboxer::RottentomatoesOnebox::FRESH_IMG
/vagrant/lib/oneboxer/rottentomatoes_onebox.rb:8: warning: previous definition of FRESH_IMG was here
/vagrant/lib/oneboxer/rottentomatoes_onebox.rb:9: warning: already initialized constant Oneboxer::RottentomatoesOnebox::POPCORN_IMG
/vagrant/lib/oneboxer/rottentomatoes_onebox.rb:9: warning: previous definition of POPCORN_IMG was here
```

*  There are 3 failing tests, however, they fail on master as well.

Thanks!