ArcpyMockup allows users without arcpy load arcapi

The ArcpyMockup instance with pretend that arcpy is present on computers
where arcpy is not available. Therefore, you can load arcapi even if you
don't have arcpy and use some of its functions that do not require
arcpy.
Merge pull request #11 from NERC-CEH/develop

ArcpyMockup allows users without arcpy load arcapi