(closes #23)

(closes #22)

ZRTP_SRTP_HASH_SHA1 was moved from public zrtp_hash_id_t to the
private srtp implementation. HMAC SHA1 constant was also renamed to
ZRTP_SRTP_HASH_HMAC_SHA1 in order to raise an error if the user tries
to use an old ZRTP_SRTP_HASH_SHA1 constant in a ZRTP session
configuration.

(closes #20)