Added and moved docs alphabet.mli
Moved bitvector C files in own directory; scripted

- Fixed issues I had with scripting bitvector in their own directory
- Added build support for vectorization; more is needed, like a configuration
  step to choose the method, and add the files to the libphyloc.clib file --can
  this be done programatically based on state in myocamlbuild.ml?