Replace autotest with guard.

Autotest was giving me gray hair.  Guard has worked seamlessly as
  expected.
To use, type guard at the command line.
There are a few OS X-specific Guardfile items that can be changed.
Add Entity and Factor.

Moving to a data structure that helps me share conventions
  across action objects.
A Factor, as currently defined, is easy to use for discrete factors,
  and I am assuming the properties hash will be useful for continuous
  factors.
The Entity extends an ActiveAttr::Model just a little, using the
  hydrate naming convention, and loading from_json, from_yaml
  and from_file.