Fixes migrations to work from an empty db and from empty.sql as well - issue #34

Fix for issue [#34](https://github.com/publiclab/plots2/issues/34)

I modified all the migrations so that doing a `rake db:migrate` will work with both an empty database as well as a database created from the empty.sql file. It will be possible from now on to create migrations in the usual way. The PR also updates the schema.rb file.

Additionally, the last migration drops any tables that aren't referenced by the rails app, cutting down the number of tables to 27 (+1 for schema_migrations).