Added a Mapping class which appeared to be missing. Can now do:

    cft.mappings.add(Mapping(...))

A couple of basic tests for this code also inserted.

Added code to cli.py to facilitate running with:

    python -m cfn_pyplates.cli

Added requirements files to assist developers using checked out code
without running setup.py.
Accidental format error introduced
Merge pull request #1 from Rethought/mapping

Addition of Mapping class