close event channel on shutdown

[zeidlermicha]

this will help jumping out of the for loop:
for event := range watcher.C()

[tslocum]

Simply closing the channel isn't quite right. Without handling this closure in the goroutines holding a reference to the channel, the application will panic when they attempt to send an event through the closed channel. Example:

goadb/device_watcher.go

Line 181 in 029cc6b

eventChan <- event