Pre 1.0

There are too many changes in this merge request. I would've tried to split them, but most of them are pretty much dependent on each other.

• Enhanced meta-programming for models
• Add plugin metaclass to keep track of plugin types
• Plugins can be imported from a python file if they're named senpy_<whatever>.py> or <whatever>_plugin.py (no need for .senpy files anymore!)
• As a result of the previous one, I refactored the code to get rid of unnecessary .senpys
• Add Box plugin (i.e. black box)
• Add SentimentBox, EmotionBox and MappingMixin
• Refactored CustomDict
• Add docstings and tests to most plugins
• Read plugin description from the docstring
• Load models, plugins and utils into the main module (senpy) (see init.py)
• Enhanced plugin development/experience with utils (easy_test, easy_serve)
• Fix bug in check_template that wouldn't check objects
• Make model defaults a private variable
• Add option to list loaded plugins in CLI
• Update docs
• Add sklearn example
• Several fixes for test_case
• Simplified test_case for different types of plugin (Sentiment, Emotion & Analysis)
• Add SenpyClientUse docs
• Change names of classes in the plugins module to avoid repetitions (we may have to revert this)
• Make subprocess log private
• Moved plugins to examples dir (in a previous commit)
• Simplify validation in parse_params
• Added convenience methods to mock requests in tests
• Change help schema to use .valid_parameters instead of .parameters, which was used in results to show parameters provided by the user.
• Add loading attributes in class
• Add ability to specify failing test cases in plugins
• Improve UI
  • Added basic parameters
  • Fixed bugs in parameter handling
  • Refactored and cleaned code