Add more usable parts

[Brian Gerkey]

Add:

• gasket thingy
• disc thingy

For each one, add reference feature as necessary to reduce orientation ambiguity. Also revisit collision bodies, considering use of a collection of simple shapes to build up to an object with a hole in it.

[Deanna Hood]

addressed by https://bitbucket.org/osrf/ariac/pull-requests/34