adding suit constants

Changes
* adding constants for suit names to PlayingCard
* fixing some pep8 errors
adding validation to PlayingCard

Changes
* adding CardException
* adding check for suit and value  on PlayingCard creation
* cleaning up comments in exceptions
refactoring suit constants
adding equality function for PlayingCard

Changes
* adding suit property to PlayingCard
* adding __eq__ to PlayingCard
* adding tests for equality and inequality