Fixes incidental incorrect hmac signature for Bitfinex

Changes the minimum width for the signature in hexadecimal base from 64
to 96, because the algorithm use 384 bit keys, this way leading zeros
don't get trimmed anymore.
Merge pull request #311 from tbosman/develop

Fixes incidental incorrect hmac signature for Bitfinex
Merge pull request #1 from timmolter/develop

Bitfinex Hmac Fix