Move set_fallback to Reactor from Handle. (#93)
Move set_fallback to Reactor from Handle. (#93)

The caller should be required to have control of a reactor when setting
it as a fallback. A `Handle` is used to pass into libraries and
functions and should not grant capability of using the associated
reactor as a fallback.

Closes #71
tokio-rs/tokio,tokio-rs/tokio