Side-effect: some constant strings are now public

- Migrated I2PActivityBase and subclasses to Fragment
- Added new MainActivity with navigation drawer to handle Fragments
- Added MainFragment to navigation drawer

Code compiles, installs and runs. Known bugs created:
- Some buttons now fail (can't use an Intent with a Fragment)
  -> Migrate usage of Intents to Fragment transactions
- New install/upgrade dialogs are commented out
  -> Migrate old Dialogs to DialogFragments

The path in project.properties will require local overriding. I tried directly
referencing the SDK with ${sdk.dir} but the build fails with an absolute path
to a library.

- Update release notes
- Lower log levels
- Spelling fixes on settings page

(for copying on build) and changed LicenseActivity to show it.

Also did some changes in AddressbookSettingsActivity; changed the way subscriptions.txt is readed and removed tabs. also changed some handlings in save function.