Add Ep.VERSION.

I've added an `Ep.VERSION` constant so that client code can easily check
to make sure that we're running an expected version of EPF.

  - Add `Ep.VERSION` to `lib/version.js`
  - Add test for `Ep.VERSION` to `test/version.em` which compares
    against the version provided in `package.json`.

I couldn't think of a nice way to dynamically change the value of
`Ep.VERSION` to match `package.json` without duck punching the build
process, so I've settled for a test which will fail the build if someone
forgets to bump it when they bump the version in `package.json`.