Merged all improvements from other branches

This pull request includes all the improvements in other branches of this library on github.

Included are: 
  * New `ProductConfigurable` class which requires the magento-webservices module by @resteve 
  * `ProductAttribute.info` method based on work by @resteve
  * `ProductAttribute.createOption` by @www-xilus-es, renamed to `ProductAttribute.addOption` to match the Magento API's name by @glyphobet
  * `ProductAttribute.removeOption` by @resteve
  * `ProductAttribute.create`, `ProductAttribute.update` by @glyphobet
  * PEP8 cleanup by @resteve
  * Fixed multicall by Ali Dropt/magento@8a5fea7f98821290e5fa61d198e8f8ea1013e6b0
  * README in markdown format by @glyphobet
  * Some minor changes and cleanup