Add transduce transform. Resolves #212.

[vqvu]

Docs adapted from @kevinbeaty's suggestion, but with the parts about resuming streams stripped out, since no transform resumes streams.

Error handling:

• If step or result throws, then the transformation stops and the error (and nil) is pushed. This matches the behavior of reduce and scan.

[svozza]

Nice! Might be useful to have a test for a transducer that terminates early like takeWhile so we're testing the res.__transducers_reduced__ branch:

var isEven = function(n) { return n % 2 == 0; };

var xf = transducers.comp(transducers.takeWhile(function(n) { return n < 5; }), transducers.filter(isEven));

_([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]).transduce(xf).toArray(function (xs) {
    test.same(xs, [2, 4])
});

[vqvu]

Done. The test just uses a take transducer.

[vqvu]

Note to self: Rebase onto master before merging.

[vqvu]

Since there's no more outstanding comments, I'll go ahead and merge this.

[vqvu]

@jeromew Can you do a grunt release:minor with the latest master? @ibash also wanted a release.

[svozza]

This is fantastic, fair play for all the work you've done lately not just on this particular PR.