Generalized WaterBox test to allow other water models to be specified.
Added subclasses for convenience construction of four- and five-site waters.
Merge pull request #42 from jchodera/improved-waterbox-testsystems

Generalized WaterBox test to allow water model to be specified. Valid choices are tip3p, tip4pew, tip5p, spce.  No AMOEBA water support yet.