Add `PostTimeout` function

[rafrombrc]

This patch makes 4 changes:

• Adds PostTimeout function that lets you tell go-notify to time out after a specific time duration for output channels that are blocking.
• Adds StopAll function that lets you unregister all channels listening for a given event.
• Moves the E_NOT_FOUND error string to a constant so the output string is only specified in a single location.
• Adds an outChans variable that is used to store the results of events[event], so the map lookup doesn't have to happen multiple times unnecessarily.

The PostTimeout and StopAll functions are both particularly useful from within test code, when you might be exercising code that sets up event listeners that are otherwise beyond control of the tests themselves.

[mreiferson]

thanks @rafrombrc I completely missed this pull request, sorry for the delay...

[mreiferson]

overall changes look good other than those few nitpicks

[rafrombrc]

Yup, good spots. Made the requested changes.

[mreiferson]

cool, thanks. do you mind squashing this down to one commit?

[mreiferson]

errr, maybe 3:

1. for the error change
2. for StopAll
3. for PostTimeout

?

[rafrombrc]

Commits squashed. First time I've used "git rebase -i"... surprisingly easy!