Call require("mock-modules").register in every mockable module.

Mocking happens only when config.constants.__MOCK__ is true.
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.
Store dirtyMocks array globally so it can survive dumpCache().

When require("mock-modules").dumpCache() is called, all mock functions
previously created continue to refer to the old dirtyMocks array.

If we replace that array with a new one, those mock functions will never
have their .mockClear() methods called again.

The upstream version of mocks.js pulls a similar global trick, and I never
understood why until now.