Add get_public_options method to BaseTransport

If the transport server wishes to prevent some server
transport options from being added to the BridgeDB then
the transport may override this method and return a
transport_options dict with the keys to be distributed.
Merge branch 'david-transport-public-options' into david-bananaphone-public-options
Only tell bridgeDB about the encodingSpec option

Use get_public_options class method to set encodingSpec
as the only server transport option that should be distributed.
Clean up code to filter server transport options
Merge branch 'david-bananaphone' into david-bananaphone-public-options

manually resolved Conflicts:
	obfsproxy/transports/bananaphone_transport.py
Remove default encodingSpec transport option
Merge branch 'david-bananaphone-public-options' into david-bananaphone

manually resolved Conflicts:
	obfsproxy/transports/bananaphone_transport.py