Add more granular date locators
Treat bool columns as discrete values for coloring
Merge pull request #254 from cosinequanon/master

Treat bool columns as discrete values for coloring
Merge pull request #256 from darora/feature/add-more-date-locators

Add more granular date breaks
WIP

Refactor ggplot, theme, and complete theme subclasses
* set default theme to theme_gray()
* use explicit api to callback to theme for applying rcParams and post_plot_callbacks
* remove __radd_ from theme subclasses
  this is pretty messy to get right better that it is only defined in the superclass due to the
  way python decides what to call when __add__, __radd__ are available for parent and child classes

Get the basic adding semantics of ggplot + theme and theme + theme
* Add element_text for themeing text
* themes can be combined iff compined in the correct order
  eg. axis_text + axis_text_x will work as expected but
      axis_text_x + axis_text will not.
* only theming text related to axis
* todo
  * more theme elements
  * more theme targets
  * proper combination of theme elements

More text theming added

TODO:
* complete rest of themes
* implement correct order of theme application regardless of theme construction
* fix python3 print error
* update images that were failing tests