Add _.collate to array.builders

[jgonggrijp]

This is the continuation of #135, which will be merged together with the current PR. @tomrogers42 will get the credit for his original work.

135 was already extensively reviewed and I have made only a few changes that I think are pretty uncontroversial:

• Updated the test infrastructure.
• Used the _.iteratee convention instead of manually implementing key/callback support. The tests still passed after this.
• Added a test to ensure that collation is not overly unstable.

I'll be merging this without additional review, though passers-by are still welcome to review after the fact.