Proper handling of @import directive inside a file read using pkg_resources

load_definitions() takes a new parameter to indicate if the file was loaded
from as a pkg_resource. In that case, related files will be opened in the
same way.

See #68 and ufo-kit/concert#94
Fix get_dimensionality for non multiplicative units and provides test cases

close #66
Fix get_dimensionality
Updated CHANGES
Preparing release 0.3.2
Back to development: 0.4
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