Add the `unit` package from OpenMM, and make sure that it works
correctly/interchangeably with OpenMM's unit package if available, and if users
only use the `unit` namespace.
Fix subtle bug in Unit where mutable sequences that are divided by their exact
unit will return a shallow copy of their contents, allowing them to be silently
changed by proxy.
dos2unix some files and other fixes.
Get rid of useless (and incorrect) shebang.
Add the beginnings of a test suite for the new unit package.
Finish the unit test suite and fix a few other small issues in the unit package.
Make sure the chemistry.units package is built and delete the numpy tests if
numpy cannot be imported.
Python 3 fixes.
Merge pull request #12 from swails/add_unit

Add unit