Initial implementation (take 2) of TrajectoryPtSamplers

DO NOT MERGE THIS PR!

This PR is for discussion only, it is not ready to be merged. Please let me know your thoughts on implementing a sampler.   This PR beings to address #18.

Some differences from my last attempt (https://github.com/dpsolomon/descartes/tree/samplers):
* Each TrajectoryPt type has its own sampler, e.g. a JointSampler for a JointTrajectoryPt.
* Each sampler type has a pure base class.
What is the difference between this sample call and the tool/wobj sample calls below?
Is calling a sample method with a sampler argument safe?  What if the sampler is initialized with a nominal/bounds that isn't at all related to the specific point.