Add Ability to Explicitly Manage Whitelisted Domains

[smcgregor]

Add a form element to the preferences dialog that will allow users to manage the automatic injection list.

The steps involved in adding basic functionality include:

1. Adding a XUL text area to the preferences window
2. Implementing the Firefox Message Manager to connect the overlay with the content scripts.
3. Add a function in privly.js that will replace the privly.privlyReferencesRegex regular expression with the message sent by the extension, then re-runs privly.js.

This is a major deliverable of Masked

The IRC nickname "irdan" is working on this functionality.

[smcgregor]

See commit.