Add support for HMAC-SHA256.

Adds functionality to send a hash-based message authentication code
(HMAC) along with the ciphertext. The USE_HMAC flag governs the use of
HMACs; if true an HMAC will be calculated and appended to the message
packet as follows:
	[aes]nonce$ciphertext$hmac[/aes]

An incoming HMAC will be checked for authenticity regardless of the
value of USE_HMAC. A warning message is printed in bold red if
validation fails.
Add support for multipart messages.

Adds code to support multipart messages; that is, messages where the
message length is longer than the allowed IRC message length.