remove duplication for class names

Reference `$self` instead of hardcoding class names (`Algorithm::AM`,
etc.)
add scores_normalized method

The probability distribution over different class labels is probably
the most basic piece of information needed from the classifier,
so add a new method to return it.
update documentation, including background/terms

Add an explanation for the terms used in the software and what
the more correct terms mean in AM. The `outcome` method does not
yet exist, and needs to be implemented in a future commit.

Generally update to make documentation clearer. Rename variables
for clarity, as well.