Add rake test task to run tests

Signed-off-by: Johnneylee Jack Rollins <Johnneylee.Rollins@gmail.com>
Add :test as default task

Signed-off-by: Johnneylee Jack Rollins <Johnneylee.Rollins@gmail.com>
Move rails initialization and task loading to task

This was changed because the goal of the test suite is to be able to be
ran quickly depending on the environment required for the tests being
ran. By moving the loading of rails to a separate rake task you gain the
ability to write commands like `rake rails stat` as a shorthand for
setting the `RAILS_ENV` environment variable to anything and then
running the normal rake task.

One caveat for the shorthand `rake rails <task>` form is that `rake
rails -T` doesn't include the rails tasks. For these situations use
`RAILS_ENV=1 rake -T`. You'll notice that you can set anything as
`RAILS_ENV`.

You may also run your test files directly with `ruby -Itest
path/to/test_file.rb`. `bundle exec` may or may not be necessary.

Lastly, poor measurements:

* 0m1.144s `bundle exec rake rails test`
* 0m0.934s `rake rails test`
* 0m0.800s `RAILS_ENV=1 bundle exec rake test`
* 0m0.600s `RAILS_ENV=1 rake`
* 0m0.580s `bundle exec rake test`
* 0m0.250s `rake test`

These measurements were taken with a single test that fails with
`assert_true false` and the currently barebones rails application.

Signed-off-by: Johnneylee Jack Rollins <Johnneylee.Rollins@gmail.com>
Make loading lib/tasks possible without Rails