ParseHelper can now parse units named like python keywords

ParseHelper gains a new class method to perform the common operation of
creating an object with a single unit and exponent 1.

Still to do:
- Do something similar for UnitRegistry.parse_expression
- Unify the __getitem__ and __getattr__ behaviour in UnitRegistry
- Parse unicode identifier

fix #69
Updated CHANGES for issue #69
Merge branch 'master' into develop