Allow Transport to be passed on initialization

Enable optional Transport for ServerProxy. This enables the
ability to set custom headers for requests, for example, X-Forwarded-For
or User-Agent, which can be required for Magento's Session Validation.

See: Mage_Core_Model_Session_Abstract_Varien::getValidatorData
Docstring documentation for transport kwarg

Add transport param to API.__init__ docstring
Merge pull request #20 from aventurella/transports

Allow xmlrpclib.Transport subclass to be passed on initialization