Added Settings Module

This adds a module with a simple interface for getting/setting
settings.  I'll use this later for the options page.
Add Storage Method Setting to Options Page

This turned out to be significantly more difficult than expected.
I've added a control on the options page to let the user switch back
and forth between using local and sync storage for their passwords.
When they choose to switch, they're also given the option to transfer
their passwords to the new storage method if desired.
Options Page Styling

Just adding some minimal styling to the options page.  Makes the tab
selector horizontal, and adds some more CSS classes to use later along
with headers for the tabs and sections.