Clean up a few imports and rename two ARC specs
Adds support for checking if a file URL exists

expect(URL).to(exist);
// or
cacheURL should exist;
Adds support for checking if a file path exists (as an NSString *)

expect(path).to(exist);
// or
path should exist;
Create caches directory if it does not exist

This is needed when we run iOS specs outside of the simulator, for
instance in Travis CI or via rake