Improvements to crypto-password.

Following the advice @abedra gave at Clojure/West '14, I have done the following things:

* Increased default iterations of PBKDF2 to 100,000
* Implemented support for HMAC (supports MD5, SHA1-512 with SHA1 being the default)

Let's all work on this together and make `crypto-password` better.

Suggestions welcome.