Wrapper tasks for :assets-dependent Jasmine tasks

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()).