This is in preparation for the upcoming Galois/Counter mode, which
conventionally has 12 bytes of IV as opposed to CBC's 16 bytes.
...Also fixes a lot of style issues, since the author finally found the
project's clang config file in the repository root :^)
This commit fixes up the following:
- HMAC should not reuse a single hasher when successively updating
- AES Key should not assume its user key is valid signed char*
- Mode should have a virtual destructor
And adds a RFC5246 padding mode, which is required for TLS
