Replace time with std::time

[serprex]

Cargo will still pull time in as a dependency at times through mio, but mio 0.6 doesn't depend on time

[dfaust]

Thank you very much. I looked through the code and it looks good.

The biggest change I could find is that the poll watcher now uses the monotonic - but not steady - std::time::Instant instead of the high-resolution performance counter from time. This shouldn't change the behavior of the poll watcher, though, because the time is only used to compare last_check time-stamps. The polling interval is still set by thread::sleep().

[passcod]

I'll release this as a patch-level change later this week, along with the deprecation from #112.