do not flood charmstore on publish

use a 500ms delay between charm-event polls

(Review request: http://reviews.vapour.ws/r/568/)
why not pollDelay = 500 * time.Miliisecond?

Also, we really want a const to be used here:

// defaultPollDelay is the default interval between charm store polls
// unless SetPollDelay is used to set a different value.
const defaultPollDelay = 500 * time.Millisecond

and then

pollDelay = defaultPollDelay

Also I'm not sure this even compiles does it?
Shouldn't it be

c.pollDelay = ....