Add Sleep future.

Example:
from cocaine.futures import Sleep
from cocaine.logging import Logger

l = Logger()

def work(request, response):
    l.info(str(time.time()))
    s_future = Sleep(0.5)
    wake = yield s_future # return after 0.5 sec
    l.info(str(time.time()))
    wake = yield s_future # return after 0.5 sec again
    l.info(str(time.time()))
    response.write("OK")
    response.close()