Automatically require js-factories when possible

I had a tough time getting chai-js-factories working on my machine, and found a few out-of-date/rough edges in the plugin. Refactored it to have clearer setup directions and to just work (TM) whenever possible.

Tested this explicitly in Node and the browser harness. I haven't tested AMD, but I don't see any reason why it shouldn't work--pretty straightforward.