Updates for gregorio-mode.el

Sorry for such a large pull request, but I have been working on a gregorio-mode myself for a few months with no progress. Seeing yours on the gregorio mailing list, I am happy to contribute to it. The only major change I made was the keybinding for gregorio-to-tex. F1 is more of a universal help menu for the modes (if a mode defines one).

Added gregorio-mode-keymap
      - Removed setting global keybindings.
      - Redefined the keybindings into a local keymap.
      - Changed keybinding for gregorio-to-tex from F1 to C-c C-e

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.