Add Dogecoin support

Added Dogecoin network protocol version numbers.
Added Digishield difficulty calculations for Dogecoin.
Dogecoin testnet difficulty reset now functions correctly.
Added initial draft of AuxPoW handling, primarily for Dogecoin at this time however
any chain following the Namecoin merged mining specification should work. AuxPoW blocks
are now identified, and the AuxPoW data validated and checked against the difficulty of
that block.