Make jest-cli a real module.

jest now has a main file which only exports. It also has bin/jest which consumes itself as a module.

This results in bin/jest being much smaller and easier to understand. It's now only responsible for finding the appropriate jest module and running it's runCLI method.

src/jest now only exports things. It exports runCLI, as it did before, but also exports TestRunner directly, should someone wish to operate jest without CLI (an important first step to running jest in a browser for browser CI testing).

the runCLI method has been simplified by moving it's steps into separate functions, reducing duplicated logic and making the chain of events a bit easier to follow.