New classes in order to use SQLite as backend for NAT rules

The current StringSet in SharedPreferences cannot comply with the new
features we'll add to orWall.
Using SQLite will allow us to provide new features like per-app Onion
Router choice, ports and so on.

Most probably the schema will be updated shortly, it just shows the
basis. Most of the fields will not be used for now, as we need some more
time in order to modify the UI and provide new options.

SharedPreferences is kept in order to set application preferences only
(stuff in settings).
new broadcast receiver: remove rules when we uninstall an app
new constants
updated classes in order to use new SQLite backend for NAT rules
using constant instead of hard-coded string
new method: import rules from SharedPreferences
Import rules from SharedPreferences, and remove entry once it's done
some corrections for new broadcast filter
corrected how db.query is called — not really clean, this thing…