#11 Added support for 256 bit HMAC

[Dylan Katz]

I am unsure if you're actively accepting patches for this project, but I figured I'd submit this anyways. This change enables support for 256 bit HMAC if JCE supports >= 256 bit AES keys (which is now default behavior).
One caveat to this is it requires larger buffer sizes for packets sent, so that is now controlled via constant in Channel, which needs to be at least 112 to support the larger MAC.