Create mock mode

[vsoch]

To better test plugins, we should allow a local or mock mode, meaning that instead of importing flux and parsing actual jobs, we return a fake set. This can be done by packaging those flux.X calls into a FluxHandle (or similar) class, and then also having a FluxMock class as an option if the user wants to mock. And then for the first, the imports for Flux will happen inside of the module functions.