implement keyboard-awareness

this completes the proposed API to provide blessings with keyboard
input. this performs the expected heavy lifting of translating special
multibyte sequences representing application keys to a unicode-derived
instance of Keystroke().

awaiting feedback from Erik Rose.

Yet to remain:
  - example code within README.rst
  - bugs ?!? 

--

I must have rebased into the sequence-awareness ?! so its too late to undo it, since i've just squashed my history. Pretty crummy advice, that rebase, now you get it all in one, just like you wanted ...