Implement PBKDF2, Scrypt, and HMAC

With Rust getting more mature, its only a matter of time until someone decides to store passwords with an application written in it (if that hasn't happened already). If someone is going to store a password, its important to protect it against offline brute force attacks. Rust already has some Digest implementations, but they are all exceptionally poor choices to encrypt a password since they are designed to run very quickly and are easily attacked with brute force methods using GPUs. The three major functions to securely store a password are PBKDF2, Scrypt, and Bcrypt. This pull request implements the first two of those. Scrypt is the most secure and is based off of PBKDF2. PBKDF2 is a standard. Bcrypt is less secure than Scrypt and isn't really a standard, so I didn't implement that function at this time. PBKDF2 requires a Pseudo Random Function to work which is generally an HMAC, so I implemented a Mac trait (Message Authentication Code) and the HMAC struct that implements that trait. I also added a new method to the Digest trait, output_bytes(), which does basically the same thing as the existing output_bits() method, but is generally more convenient to use.

I didn't implement any benchmarks for the new functions since Key Derivation Functions are slow by design and Rust's benchmarking functions expect each iteration to be very fast. So, the benchmark would have to test exceptionally weak parameter values to work at all which would mean that the benchmark would be testing configurations that no one should really be running in practice.