Ensure that 'dev' and 'test' sqlite databases live in different files

Previously, when running a rake task, the database.yml would
be generated such that both 'dev' and 'test' databases used
the same file, like db/development.sqlite3. This prevented
tasks that crossed RAILS_ENV boundaries, like app:db:test:prepare,
from correctly finding the test database.

Additionally, running db:schema:load and db:test:prepare in the
same 'rake' seemed to not actually load the schema, so i've
split them up in the README.

Fixes #22