Add second mask argument to printBonds to restrict bond printing. Addresses
issue #4 on Github partially (still need to do printAngles and printDihedrals).
Add two additional masks to the printAngles action to take another step in
implementing enhancement #4 from Github.
Expand printDihedrals to implement github issue #4.  Now all that's left are
tests.
Fix printAngles to recognize reverse order correctly -- drastically simplified
(faulty) logic.
Fix printDihedrals for reverse-order masks and simplify logic.
Expand the test cases for printBonds, printAngles, and printDihedrals to test
all aspects of the new functionality.
Simplify printBonds logic.
Expand a docstring.
Fix typo.
Merge pull request #23 from swails/expand_prints

Expand prints.  This implements #4