bufferCount implementation for flyd streams.
Buffers the source stream, and emits when the buffer is size bufferSize, and starts a new buffer each bufferEvery.
(Number [Number] Stream) -> Stream Buffer
a: {1-2-3------4----5---6-----7---}
bufferCount(3, 2, a): {----.-----------.---------.---}
[1,2,3] [3,4,5] [5,6,7]
var stream$ = flyd.strem()
var buffer32$ = bufferCount(3, 2, stream$)
stream$(1)
assert.deepEqual(buffer32$(), undefined)
stream$(2)(3)
assert.deepEqual(buffer32$(), [1, 2, 3])
stream$(4)(5)
assert.deepEqual(buffer32$(), [3, 4, 5])