Change queue service to automatically base-64 encode all messages that a...

...re sent to Azure.  When reading a message from Azure, the unprocessed base-64 message is stored in the raw_string field of the message object.  You can call message.as_text() and message.as_bytes() to decode the value to the desired type.  This is a breaking change!  This is required to achieve parity with the other Azure SDKs (C#, Node.js, etc)

#150 Storage queue service should automatically encode/decode base 64