Several fixes

* Refactored BaseModel for efficiency
* Added plugin metaclass to keep track of plugin types
* Moved plugins to examples dir (in a previous commit)
* Simplified validation in parse_params
* Added convenience methods to mock requests in tests
* Changed help schema to use `.valid_parameters` instead of `.parameters`,
which was used in results to show parameters provided by the user.
* Improved UI
    * Added basic parameters
    * Fixed bugs in parameter handling
    * Refactored and cleaned code

requirements.txt

@@ -3,7 +3,6 @@ requests>=2.4.1
 tornado>=4.4.3
 PyLD>=0.6.5
 nltk
-six
 future
 jsonschema
 jsonref