Rake by default imports all .rake files in the rakelib dir, so we can
use that rather than doing our own import loop.

We used to specify specific rake test tasks so that we could run all of
them even if early ones failed. However, that meant that as new tasks
were added, they weren't being run on jenkins.

Now, there is a facility in the rake scripts so that tests can run using
the test_sh function, which will delay failure until the end of the rake
run, unless the TESTS_FAIL_FAST environment variable is set.

Furthermore, this reorganizes the jasmine test tasks so that we can run
those as part of `rake test` as well.

Using `task()` reopens the definition of the task, and all we really
need to do is get a reference to the task itself to invoke it.

The :browse_jasmine_<system> and :phantomjs_jasmine_<system> tasks
depend on the :assets task, which needs to receive both :system and
:env arguments. Therefore, new tasks (:browse_jasmine and
:phantomjs_jasmine) are created that do take :system and :env args.
The old :browse_jasmine_<system> and :phantomjs_jasmine_<system>
tasks now wrap the new tasks, passing in <system> as an argument
and 'jasmine' (for :env, since it's hardcoded to 'jasmine' in
django_for_jasmine()).