Fix "zeus test"

Attempt to fix currently failing `zeus test (rspec, testrb)` command (#213):

1. `Rake::Task['test']` fails to invoke the `test` task. To fix this, use `Rake::Application` to load the app's Rakefile, and then invoke the `test` task.
2. After fixing the above, `zeus test` on an app using RSpec still does not work as expected because the test command falls through to `Zeus::M`. To fix, invoke the RSpec runner on the following condition: 

  1. no arguments are passed in `zeus test`, and
  2. `defined?(RSpec) == true`

  This means that if RSpec and *other* testing frameworks are in use (#149), `zeus test` will run the RSpec suite by default. To run another framework's suites, the user will have to run `zeus test <path_to_other_suite>`. Hopefully this is reasonable.