Call require("mock-modules").register in every mockable module.
Enable module.exports mocking in react-test.js.

We don't currently attempt to mock modules automatically, but we do
respect require("mock-modules").mock, .dontMock, and .dumpCache.

I'm going to keep investigating auto-mocking, since that would move us
much closer to the behavior used within Facebook.

Closes #154.
Closes #155.
Only mock when config.constants.__MOCK__ is true.

The meaning of __DEV__ is pretty overloaded, so depending on that was
asking for confusion.