links has become forms

[rwb27]

Currently, td_parser fails when looking at a new LabThing using the current version of the Python server. I believe this is because it's expecting to find links in a list called links for each affordance, but they are actually in a list called forms. This is the same API change addressed by #4, in a different place in the client code.

[rwb27]

This could be fixed by changing line 14 of affordances.py to use forms instead of links but of course that's not backwards-compatible. Doing so with a fallback to links would work, I think.