Adds unittests based on input type

*fixes error in _prob function (found via unittest)
Adds to-dos and the beginning of an explanation of Bayesian classifiers to the README
Adds a derive_probability method, which returns probabilities based on Bayes Theorem

* alters _prob method to revert to this, if necessary
Adds predict function

* removes code that had been commented out
* adds sample code to main
Changes count from a dictionary to a 2-D array

* Adds self._words, used to get the index of a word
* parameterizes the not-found function in the prob function
	- allows the user to specify how she wants to deal with words that have not yet been encountered
Fixes bug in basic probability calculation