Fix comma when useCommaKey=False

Fix the _onKeyDown function to preventDefault on tab, esc, and enter not
on comma. Keycode was defined incorrectly.
Add Keycode lookup

Add KEYCODE variable with lookup values for Comma, Esc, Tab, Space, etc
and used it to refactor the hard coded keycodes through the code.
Merge pull request #140 from kevinmckinley/master

Fix bug: comma key ignored when useCommaKey=false