handle order=None
fixed displaying of errors
added translation for variant's image update button, fixed typo, changed indents
Modified property handlng for variants
modified property handling for variants
make tests pass
moved caching helpers to caching utils, added cache invalidation for properties
handle empty sorting
added can_be_paid method for order
fixed filters - omit empty productpropertyvalue
fix for prices/taxes lower than 0
handle multipleobjectsreturned
fixed setting sorting in session; fixed handling of m2m add
use responseservererror
Fix attribute error from issue: #62
fixed ecommerce tracking code
Merge pull request #59 from pigletto/master

Fixed test cases, improved properties support for variants