Added removeFile method

In order to remove least recently used images I've appended a removeFile method.
Merge pull request #33 from airamrguez/master

Thanks for this.

It looks like the management of the cache space is a new concern as this week someone added a method to automatically clear the cache when its size is bigger than a given limit.

I will soon need to add documentation and unit tests (and bump version number) for all those new methods.

Cheers.