added comprehensive sequence test suite generator

time to refactor these specs!
renamed toBeASequence to toBeInstanceOf
beefed up comprehensive test suite generator

Now it supports aliases and does some more lazy checks, and
applies all tests to all sequence types!
remove redundant identity function from spec_helper.js
removed a bunch of (now) redundant specs
updated test suite generator a bit, added Lazy.clone
renamed testSequence to comprehensiveSequenceTest
WIP
return result of parent.each for #take, #drop, #takeWhile and #dropWhile
fixed behavior of #reduce on methods chained from #async
added #isAsync convenience method

I'm not 100% sure this is a good idea; it made sense for a little
experimenting I wanted to do, and I figured it couldn't hurt to expose
it. I might remove it, though.
fixed comprehensive tests to do the right thing w.r.t. async().take(2)

also added comprehensive filter specs
more comprehensive filter specs