Fraction: Add section headers in anticipation of what is to come
Add some integer math to Fraction

I haven't started working with Fraction-on-Fraction operations; that
will be fun indeed.

Note that I use getImproperNumerator() to handle cases where a unit has
not been extracted (i.e. chained operations; an example is in the < 0
case of the pow method)
Add a mean-sounding note saying that getImproperNumerator is the law
Add additive Fraction operations
Remove some of the clunky language from the Javadoc

Specifically, removed "returns" and "a new Fraction representing." The
fact that this is a non-static return method should be evident from the
header; the "a new Fraction representing" is included in @return.
Add multiplicative Fraction-on-Fraction operations
Add divide-by-0 sanity checks to Fraction
Add whole-number Fraction constructor
Document Fraction constructors