Revert extractValue function so that it allows users to specify an arbit...

https://github.com/ghiden/angucomplete/commit/b225cb2f900f43cf0ddf65545f031536c1d0a1a7
As stevenharman pointed out, my last attempt to remove all eval broke some people's code. So I revert the extractValue function back. I added some tests so that if someone tries to refactor it, these tests should pass too.