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.
See comment about CartesianPtSampler above.