Updated the extests to easily support image tolerances.

This pull request makes the interface easier for developers to change the tolerance of the extest images. Previously this was not possible and as a result @ajdawson was not able to test his gallery addition in a previous ticket.

I believe there is work to improve the accessibility of the examples, and to make the extests become auto-discovered, rather than enumerating them manually, but that is for another ticket. 

@ajdawson - if your able to run this and tell me the RMS difference values your getting, I will set them in the tests (I might do that in a subsequent PR).