Clean up API

[nickbattam]

The API contains a lot of repetition of words:
e.g. in cssgen.borders.OpiBorderRenderer
self._text_renderer = text_renderer
self._text_renderer.render(...)
and in cssgen.render.get_opi_renderer()
tr = OpiTextRenderer()
wr = OpiWidgetRenderer(tr)
cr = colors.OpiColorRenderer()
wr.add_renderer('background_color', cr)
This makes the API unnecessarily verbose.

The class, module and package names are not always self explanatory.

Renaming some classes, methods and package could simplify this, e.g. <renderer>.add_renderer() to <renderer>.add() and make it simpler to understand.

[mfurseman]

I also found adding actions to be verbose and repetitive. We have

class ActionWidget
    def add_something_command(<something command args>)
        self.actions.append(actions.SomethingCommandAction(<something command args>)

Do we need to have 'Action' in the class name as well as the module name. If we're going to add actions to widgets, and there is a class that represents that action, why not pass an instance of that class to represent the action, instead of some bunch of arguments?

[willrogers]

Other proposed changes:

• Add docstrings
• Add some developer documentation
• Add methods to automatically load font and color files and add them into the module namespace
• Allow creating a named Color without specifiying r,g and b
• Require lxml in setup.py.