Basic symmetric en- and decryption

First pull request was wrong rebased but now it is how it should:
symmetric encryption, decryption, javadoc, and two seperate tests
I don't know how useful the encryption test is, I guess one test with a more general name would do it as well.
#42 
No, we can't use the test vectors, because in our API we can't control the nonce. Just checking that decrypt then encrypt = identity will have to be enough.