Add intensity parameter (-i 0:31)

Like cgminer, the value equals to 1 << n
if 0, we keep the default value defined in algo (19 for Xn algos)

19 = 524288 threads per gpu call

GTX 970 and 980 handle a higher number of threads compared to the 750 Ti

Signed-off-by: Tanguy Pruvot <tanguy.pruvot@gmail.com>
Handle intensity param in all algos

and add a check related to start/max nounce params
update readme