Improved error messages on faulty definition files. Now most errors are raised instead of logged.
Fixed for Python 2.6 compatibility
Fix typo in tutorial string representation example
Merge branch 'docs/tutorial_html_repr' of git://github.com/aepsil0n/pint into aepsil0n-docs/tutorial_html_repr
Implemented case_sensitive argument for parse_expression

The argument is added to:
- parse_expression
- get_name
- parse_unit_name

It also adds a private dictionary to UnitRegistry (`_units_casei`
that maps units names (in lower case) to a set of
real unit names (in the correct case)

See #105