Adds Alt+Shift+M keyboard shortcut for invoking popup browser action

Using the Alt+Shift level here to not collide with default Chrome
key shortcuts that consistently use Ctrl/Command+Shift.
I chose M because it is easily reachable on the keyboard and because
it allows me to remember the shortcut by thinking of calling the
password[M]aker. Also, using Alt+Shift+P seemed a bit odd to me as
usually browser key commands including P invoke functions to do with
printing.
Adds password field's fill-on-enter behaviour to confirmation field

So that people who are not using the "save master password hash"
feature get the same behaviour from the confirmation textfield, as
users who save their hashes get from the password textfield. Resulting
in a fast and consistent keyboard navigation experience.
Adds Information sections to extension options

For now the section contains some fundamental information on how to navigate
PasswordMaker Pro using a keyboard
Merge pull request #82 from oolongbrothers/keyboard_navigation

Keyboard navigation