Added fragment_exist? method

I've added a new patch at Lighthouse: http://rails.lighthouseapp.com/projects/8994/tickets/203

It adds a fragment_exist? method in addition to (read_fragment and
write_fragment). This is most useful when using file_store cause You
reduce the IO operations and it's better then doing:

read_fragment(path).nil?

To this work properly, I also added an exist? method in all cache
stores.

Tests are included. =)