WIP: Add Nettle Ciphers

By borrowing heavily/copying and pasting the hash.jl 'autowrapping' I now have Nettle's cipher support working in nettle.jl. A few caveats:
- No tests yet (hence the WIP) but AES128 works for me on http://cryptopals.com/sets/1/challenges/7/
- No documentation (README)
- The code seems almost similar enough to hash.jl that it seems the two could be factored out somehow, but I figured it'd be better to get something working first. Plus see next point
- Some ciphers allow a wide range of key sizes while others are picky. Since this varies by cipher and is only encoded by #defines in the nettle I don't see an easy way of capturing this. Currently I'm requiring the user specified key to be same length as the "recommended" key size.