Command execution timeout control

[bitprophet]

Fabric 1 had a feature which (IIRC) used a threading Timer to raise an exception (which I think then just turned into ye olde crappy abort()) when a command execution took longer than the user desired.

This should live at the Invoke layer for v2, even if Fabric's Runner subclass needs hooks to implement it (though I suspect it may not, as long as we interrupt the remote process correctly when we time out - something that may or may not be necessary locally, I'm not positive.)

[bitprophet]

See #645 and commentary. This is about to get merged.