Missing one-off en/decrypt methods

[mikeboers]

See #3 for genesis of this issue.

TomCrypt has a number of "memory" functions to encrypt a message without setting up a cipher and walking through all of the stages. We should expose a similar functionality in order to assist in the interoperability with other major crypto libraries.

The straight encryption modes are simple enough, but we need to do some research as to how other libraries handle the MACs generated by cipher auth modes. In the case of the linked issue the other library simply concatenates the MAC onto the end of the string.

The proposed interface is:

from tomcrypt.cipher import aes
aes.encrypt("my message", key=b"0123456789abcdef", iv=b"...", mode="ctr", ...) # Returns the encrypted message.