Fixes for Azure SDK bugs - https://github.com/WindowsAzure/azure-sdk-for-python
117 - Service bus authorization code doesn't go through the proxy server
118 - Proxy doesn't support specifying credentials

Moved service bus helper functions from servicebus/__init__.py to servicebus/servicebusservice.py so the helpers can have access to the class' state.
This allows reusing the connection code from httpclient.py so we can benefit from proxy support and avoid the duplicated code dealing with creating the connection.
ServiceBusService __init__ and with_filter member methods were moved to the top of the class without modifications.
Added user and password parameters so we can use proxies that require authorization. Initial idea was to allow user to directly specify the headers to use when calling set_proxy, but due to the nature of the Windows API, decided to go with the simpler user/password parameters. This is actually simpler for the user, as it hides http details from them, and is sufficient to solve the current issue. If they ever need more headers, we can add more parameters.