Use sha224 instead of md5 to key the FileCache

MD5 is vulnerable to collision attacks which it may be possible
to use to attack a project using CacheControl. There are no
known theoretical or practical attacks against the sha-2 family of
hash functions so use one of them instead. sha224 is selected due
to the smaller digest size which will be less likely to hit edge
cases in filename sizes.
Merge pull request #21 from dstufft/use-sha-2

Use sha224 instead of md5 to key the FileCache

Note: This does raise the size from 32 to 56 characters. That is not a huge change, but in case it is, just subclass the FileCache and override the hashing as needed.