Make it easier to write unit test on a plugin

See test/weather_test.rb for an example
Add StubPlugin::load_plugin()

The class method returns the instance of the loaded plugin
instead of its class like StubPlugin::new_plugin()
Make conf in plugin configurable

Add []= and [] methods to ConfStub instances
Make options changable
Merge pull request #400 from zunda/better_plugin_test_pr

Make it easier to write unit test on a plugin