Add 'alias' (IMDb lookup) endpoint

Rotten Tomatoes allows for lookups by IMDb ID, so I went ahead and added that line to the available endpoints. 

It's a little quirky: `id` needs to be numeric (so strip the first two letters) and `type` should be set to 'imdb' (and there are no other options, currently). Here's a rough example:

`rt.movieAlias({ id: 'tt0051189'.substring(2), type: 'imdb' }, function(err, res){ /* etc */ });`