added statistics to status lines

stats:
	avg seconds / block; number of blocks in last hour, 24 hours
	avg shares / second; shares in last minute, 30 minutes, hour, 24 hours
	zero-fee PPS value; PPS in last 30 minutes, hour, and 24 hours
	zero-fee POT value; POT in last 30 minutes, hour, and 24 hours; best POT share and value
	best share difficulty, average share difficulty, average target difficulty

Individual "Accepted" messages show decimal estimate of share and target difficulty.  (Rather than int.)