Refactor and add tests for plugins.py

Improvement to the tests currently being run on ``plugins.py``

related #644 
It appears that the plugin_object never has a config attribute. Hence the assertions are never called. I am looking for an example of a plugin loading a plugin configuration with ``plugin_object.load_config()``. However, ``freeseer/src$ grep -r load_config\(.*,.*,.*\) freeseer/plugins/`` shows that there are no plugins who use the config arguement on the load_config function. (It should be the third value).