Added a new function ProbableBiTwinChainTestFast() for checking BiTwin candidates.

This function checks the numbers in the correct and optimal order.
The function determines the chain length accurately even for short chains unlike the previous implementation.

As a consequence, the primes/h statistics are now fixed.
Primes/h is now about 10% of tests/h, which matches the probability of them being prime.
few smaller adjustments to the cmake script
added mining code, but it's not working yet

at least: compilable :-)
Merge branch 'master' of https://github.com/mikaelh2/primecoin into experimental