HNKCache disk category

Limits scope and allow unit testing.
Remove unnecessary code

Image decompression makes this unnecessary.
Move path methods to disk category

And remove dependency between disk cache and memory cache.
removeFileForKey:format
setDiskImage:forKey:format:

For consistency with image cache.
Remove removeFileForKey:format:
Remove keyFromPath: from interface
Move registerFormatWithSize: to HNKCache+TestUtils
Only increase disk size if write was successful
Test for #23.
Use MD5 if key is too long for file name. Fixes #23.
Further setDiskImage:forKey:format: tests
Use extended file attributes to save key
Cancel request on tearDown