Optimized wait generators.

[jcbertin]

Removed usage of the ** operator in expo() generator.
Fixed default min_value for decay() generator: saved a test in the limit condition.
Saved a couple of tests for limit condition in all generators.

[yxtay]

Following what's done in expo to remove use of **.

Suggested change

	t = 0
	while True:
	a = initial_value
	while a > min_value:
	yield a
	t += 1
	a = initial_value * math.e ** (-t * decay_factor)
	if min_value is None or a > min_value:
	yield a
	t += 1
	else:
	yield min_value
	while True:
	yield min_value
	a = initial_value
	while a > min_value:
	yield a
	a /= math.exp(decay_factor)
	while True:
	yield min_value

Alternatively for line 57.

        a *= math.exp(-decay_factor)