Addition of Mapping class and minor amends for developer convenience

    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.