Keyboard navigation

Hi!

I added two small things that I - as a keyboard-navigating user - felt were missing:
* Ability to invoke the browser action popup with a key combination
* Ability to fill password field by pressing enter in the confirmation textfield

These changes allow for a keyboard-only workflow from the the username field of a web service to being logged in to the service (while the "save master password hash" option is disabled):
* Enter username in the service's username field
* Push tab, focussing the service's password field
* Push Alt+Shift+M, invoking PassworkMaker Pro popup and focussing its password field
* Enter master password
* Push enter, focussing on the confirmation field
* Enter master password again
* Push enter again, closing popup and focussing service's password field, readily filled
* Push enter, sending the login request

If you like the additions, perhaps it would make sense to mention the browser action keyboard shortcut to the extension description.

I tested on Chromium 20 and 23 on Ubuntu 12.04.

++Alex