Cleaned up code to follow typical elisp package style.

Added gregorio-mode-keymap
      - Removed setting global keybindings.
      - Redefined the keybindings into a local keymap.

Added gregorio-mode-hook var.

Converted most setq's into defvar's.
      - Allows the vars to have default values and a docstring.

Changed transpose-region-{up,down} functions.
      - Renamed to gregorio-transpose-region-{up.down}
      - Simplified the actual changing of the notes.
      - Added optional use of a prefix arg to transpose N steps.