Added further diagnostic detail in case of transaction ID clash.
Correct format of AuxPow coinbase script.

AuxPoW coinbase scripts now include block height as per BIP0034, which also
resolves issue of transactions being generated which collide.
Correct AuxPoW coinbase script concatenation operators.

MakeCoinbaseWithAux() was incorrectly using << to add a CScript onto a
CScript instead of the + operator. Operators corrected and structure
now more closely reflects coinbase script assembly done elsewhere.
Merge pull request #613 from rnicoll/1.8-dev-coinbase

Correct format of AuxPoW coinbase script