The tens value was wrong for values of 20+, as reported in #230.
It should be 10*costTens not 10^costTens. This wasn't detected because
the values are the same when costTens is 1, and using high cost values
is rare with bcrypt because of the performance hit.
Also added a simple hash and validate test since the KAT tests only do
validation. This doesn't cover this bug since the cost value is too
high to include in the test. It allows similar issues to be tested
locally though.
Makes docs and code consistent - the code now generates hashes
with the "2b" prefix instead of "2a". Shouldn't make any difference
in practice since previously generated hashes should still validate.
