timer: Rename sleep to delay_for, reexport from tokio

[Ralith]

Motivation

Expose convenience function for Duration-based waiting

Solution

Renames sleep to delay_for for consistency and reexports from tokio::timer. Additionally, the docs are tweaked to mention "sleep" for better discoverability. We considered standardizing on the sleep name instead, but Sleep as a type makes for an awkward noun.

[carllerche]

Thanks 👍