The abstract_clock is now templated on a type meeting the requirements of
the Clock concept. It inherits the nested types of the Clock on which it
is based. This resolves a problem with the original design which broke the
type-safety of time_point from different abstract clocks.
* Header-only!
* No external dependencies or other beast modules
* Compilation options allow for:
- Stand-alone application to run a single test suite
- Stand-alone application to run a set of test suites
- Global suite of tests inline with the host application
- Disable test suite generation completely
* Existing tests reworked to use the new classes
