Import Vince Durham's aux proof of work implementation.
Support auxillary proof of work.

Rebased from 1.7.1 into 1.7.2, moved AuxPoW checks from AcceptBlock()
into AcceptBlockHeader()
Enable RPC commands for mining aux proof of work blocks.
Replace getauxblock CBigNum with CScriptNum
send auxpow debug info to debug.log instead of stdout
extracted auxpow consts

set testnet start of auxpow to block 158100
Set chain id to 0x0062

0x0062 is 98 in decimal. Dedicated to Josh Wise.
Make version check in UpdateTip() consider auxpow block version
Merge pull request #584 from patricklodder/1.8-dev-auxpow

Enable auxpow for Dogecoin