Changes

• Implemented timeout_at(deadline) (#117)

• Supported timeout.deadline and timeout.expired properties.

• Drooped timeout.remaining property: it can be calculated as
  timeout.deadline - loop.time()

• Dropped timeout.timeout property that returns a relative timeout based on the
  timeout object creation time; the absolute timeout.deadline should be used
  instead.

• Added the deadline modification methods: timeout.reject(),
  timeout.shift(delay), timeout.update(deadline).

• Deprecated synchronous context manager usage