Added contains() method

This is a much faster way of checking if a given key exists in the cache.

In a demanding use-case, scrolling very quickly, where every item in a ListView must check to see if it's object's key exists in the DiskCache:
Some basic profiling showed this method taking ~3ms vs using ```get( key )``` taking on average ~70ms. I believe this is because ```get()``` is synchronized.

It should be safe to use ```contains()``` unsynchronized like this.

This addresses issue #54