Categories listed first have higher priorities and
the same applies to each item within a category.

Bugs
----
* Run-time stats and Async I/O don't play nice.

Feature
-------
* User provided dictionary file.
* User configurable alphabet.
* Password prefix (known begining string).
* Add MD4, SHA256, SHA384, and SHA512

Enhancement
-----------
* Parallelism for multi core/cpu systems (this should be #1 item in todo).
* Cygwin.
* FreeBSD / OpenBSD.
* Solaris.
* Find performance improvements.
* Change signal() to sigaction().
