Deprecate bcrypt port mechanism and wrap nif calls using poolboy.

This is primarily for discussion.

The primary usage of bcrypt is for the nif worker. However, the nif worker is a single gen_server which only responds to gen_server calls so the library can't take advantage of more than one core at a time.

I've integrated poolboy here and deprecated the bcrypt_pool which hasn't performed particularly well in my tests.

Also, I noticed that calling gen_salt without passing the log rounds parameter explicitly would default to infinity as opposed to what was set in the application config.