Added backport of unit tests from -dev branch.
Changes to product of q factors calculations to fix unit tests.
Changes to accept-extensions handling to fix unit tests.
Fix to returns on parsing error to make returned values consistant.
Change to storing of quality factors to ensure that the calculated values are
always returned as a string with non integer q factors prefixed with a 0.
Added new version number, dates to head of .php files.
Updated changelog to include changes in this version.