Update .travis.yml
better handling of current values in pool/flatMap

no need for Kefir.once() now
remove Kefir.once(x)
refactor the way listeners stored and called
support for arrays as returned value in flatMap's map fn
fix chaining methods call support
optimize subscribers calling, refactoring
minor refactoring
add `Kefir.fromCallback()`
remove backlog
add Yandex.Metrika counter for docs
add .debounce(wait[, options])
add $.fn.asKefirProperty(eventName[, selector], getter)
minor docs improvements
refactor; add Kefir.cocnat(); drop array support in .flatMap()
minor docs improvements
tiny docs improvement
add .flatMap variations

.flatMapLatest
.flatMapFirst
.flatMapConcat
.flatMapWithConcurrencyLimit
docs
update memory tests results