Merge pull request #30 from mhvk/units-refactor-quantity

Addition of most sensible ufuncs
Ufuncs where input is scaled rather than output being recalculated
Fix behaviour for ufunc(q, out=q)
Made output check a bit simpler
Fairly complete implementation, with inplace working correctly
Refactor __array_prepare__ (for single-argument ufuncs) to use helper functions that live in a separate file
Refactor __array_prepare__ for two-argument ufuncs
Refactored __array_wrap__ and removed unused code from __array_prepare__
Mark in-place tests as known failures
Added a note about where the idea for quantity_helper.py came from
Revert the addition of the copy= option in the ``to`` method
Fix test failures with Numpy < 1.7
Fix test failure with Python 2.6