Release version 0.4.3

* Fixed issue with pip + virtualenv.
* Fixed missing tests dir error while building package.
* Don't raise exception when the error reported by UPS is a Warning.
* VoidShipmentRequest initial implementation
* Send back the full request message.
* Added PackageServiceOptions tag.
* Code is now pep8 compliant
* New class attribute 'return_xml'.
Merge branch 'release/0.4.3'