SpatiaLite seed database

The database file includes empty city, location, and favorite tables.
"init.sh" downloads GeoNames data and imports it into the database,
placing the result in the assets/ directory.
DatabaseHelper to initialize/open database
Initial implementation of {City,Location}Adapter

These classes implement some queries over their respective tables.
A small amount of test/demo code is in MainActivity.
Refactor and document database code

DatabaseHelper is a singleton; LocationAdapter.insertLocation takes an
android.location.Location instead of individual values;
CityAdapter.getNearestCity takes maximum distance as a parameter.
Merge pull request #2 from coyotebush/database

Database setup