Deprecate accidental leaking of private API

[EricCousineau-TRI]

Relates #30, #14

It's hard to know what you can really change in the API since some of the internal implementation bits leaked out into the public namespace.

Ideally, it'd be nice to deprecate those private bits, and simplify the public interface, and do a bit more testing on it.

\cc @clalancette

[eacousineau]

Proposed steps:

• Ensure that urdf.py (and sdf.py) do not transitively import symbols from xml_reflection, and try to handle class variabes.
• Deprecate xml_reflection as a whole.
• Once deprecation period has passed, delete deprecated members.