Changes

Changes since 0.4.2:

* fixed problem with ClkGen (wouldn't work with a 1,1 clock)
* fixed BitVector xor problem
* Added unit tests for Bit, BitVector, Signal
* Changed StateType to the more generic EnumType
* Changed TestBench module name to Simulator (step is in simulator)
* Changed Simulator::step so that it takes an optional code block
* Updated docs to reflect changes
* Updated UsersGuide 
* fixed some other bugs...

Changes since 0.3.0:

* No new's is good new's:  Used to be you had to do things like:
    sig = Signal.new(Bit.new('0'))
  Now, I've added some helper methods so that you can do:
    sig = Signal(Bit('0'))
  ... which saves a good bit of typing.
* The Bit and BitVector classes have pretty much been rewritten and 
  (hopefully) improved.  I've changed the operators so that they return
  new objects.  I've also endeavored to make the operators more polymorphic
  so that you can add an Integer to a BitVector, for example.
* created a user's guide and much improved the website.
* added a ClkGen class to make it easier to create clock signals
* added a StateType class which is kind of like an enumerated state type.