This commit removes the `Reactor::run` method which has previously been used to
execute futures and turn the reactor at the same time. The tests/examples made
heavy usage of this method but they have now all temporarily moved to `wait()`
until the futures dependency is upgraded. In the meantime this'll allow us to
further trim down the `Reactor` APIs to their final state.
In accordance with tokio-rs/tokio-rfcs#3, the executor functionality of
Tokio is being removed and will be relocated into futures-rs as a
"current thread" executor.
This PR removes task execution from the code base. As a temporary
mesure, all examples and tests are switched to using CpuPool.
Depends on #19.
Alex Crichton