Add support for Diamond v2 (groestl)

Diamond has switched to groestl some months ago and there is still no support in (official) sgminer.
Using the groestlcoin kernel is inappropriate, because unlike Diamond, GroestlCoin uses single sha256 hashes for transactions and therefore Diamond stratum does not work.

This change adds a new kernel (-k diamond) and a new optimized diamond.cl file. If the format of this optimized kernel is not appropriate, it could be replaced with the groestlcoin.cl file. Having a separate kernel file for Diamond is a good idea as in future other optimizations might further divert it from GroestlCoin.