Added check for order in _ok_for_muldiv. Clean up. Improved doc strings.
Added test for multiplication/division of scalar and Quantity

- fixed bug is _ok_for_muldiv from last commit
- Fixed convert: It did not raise an error for offset units with broken
  orders in range ]0;1[.
Change: Converted functions to Quantity methods/properties

Also reorganized code in parameterized tests.
Improved exponentiation-methods of _Quantity and added __rpow__

- better handling of numpy arrays
- correct handling of offset units
- Q_(0, 'degC') raises no longer a ZeroDivisionError
- Q_(5, 'degC')**1 returns Q_(5, 'degC'). It does no longer convert to kelvin.
- Removed corresponding test that relied on the conversion for Q**1.
- Several new tests added.
- This version was used to create a table comparing the new behavior to
  pint-0.5.