return value of wrapped specifier in `it`

Mocha sometimes delegates back to itself internally. For example
`it.only` invokes `it`. When running with modules, this is not a
problem, since ember mocha's `it` does not override `mocha.it`. However,
when running globalized, `mocha.it` is replaced, and so `mocha.it.only`
will call ember-mocha `it`.
add global shim for ember

the `ember-mocha` module lists ember as a dependency, and so you get a
runtime error when you try to eval ember-mocha.js

This makes ember available as a dependency.
Merge pull request #5 from cowboyd/globalized-fixes

Globalized fixes