Add Skein as a stream cipher.
Move the Skein FFI stuff into Crypto.Threefish.Skein.Internal.
Ensure that users can't accidentally create wrongly sized blocks.
Make SkeinGen an instance of CryptoRandomGen.
Add Skein as a key derivation function.
Get rid of the nonexhaustive case warning for StreamCipher.
Don't truncate cryptotext to 256 byte boundaries in StreamCipher.
Add convenience functions for authenticated encryption.
Use lazy bytestrings for hashing.

This turns out to be slightly faster and a lot more memory efficient for
large files.
Remove nonce argument from hash256, since nothing uses it anyway.

Nonce-needing functions work directly with the internals instead.
Allocate enough memory for Skein output, even if outblocks > 1.
Update README.
Update version and description in cabal file.
Add source-repository section to cabal file.