Fix Group.get_device and refactor DeviceProxy initialization

This follows up on issue #21 (closed) and PR #35.

The fix implemented in PR #35 causes Group.get_device to instantiate a new Tango::DeviceProxy for no reason. In order to get everything to work properly, I had to refactor __DeviceProxy__init__ and add a proxy._initialized flag, so it is possible to differentiate proxies that have gone through the python initialization and those that haven't.