add appnope to external
use QSocketNotifier, not poll

avoids having to disable App Nap outright.

Also use appnope context manager to disable App Nap on OS X 10.9 while processing events.

Adds IPKernel._darwin_app_nap flag, so that working with App Nap can be disabled, in case appnope causes problems for some.

App Nap is just disabled outright on wx.