Fixed Symmetric Key Encryption/Decryption

[wendtr]

So I noticed that symmetric key encryption was not working, even though the functions existed. This fix that I put in isn't necessarily the greatest, but it allows you to use the AES encryption/decryption functions for CEK. I know this might not necessarily be the best way to do so, but it is currently a quick fix for anyone who wants to use symmetric crypto with your library.
In addition, I created a simple test in the test suite.

sregister is the other contributor to this, although it may not reflect so in the commits.

[yuriikonovaliuk]

@wendtr This is not right way to do it. A(128|192|256)CBC algorithms are not allowed for key encryption (refer to http://tools.ietf.org/html/rfc7518#section-4.1). AES encryption can be used for key wrapping but there are dedicated AES algorithms (A(128|192|256)KW. Refer to http://tools.ietf.org/html/rfc3394#section-2.2) which are not currently implemented in the library.

[wendtr]

Right after sending that pull request we realized we implemented everything poorly, and that it did not satisfy any of the algorithms supported by JWE. What we were looking to do was use Direct Symmetric Encryption (RFC 7518, Section 4.1, 4.5). I believe our current implementation of Direct Encryption implements this. The code is not implemented in the best way (uses an If-Else statement), but it could be cleaned up possibly.