Add Dogecoin support

Remove network protocol version setting so it uses value from Bitcoin-ruby.
Added Digishield difficulty calculations for Dogecoin.
Dogecoin testnet difficulty reset now functions correctly.
Updated minimum work that can be required for a block to handle Dogecoin Digishield blocks.
Disabled expectation of block v2 for Dogecoin.
Expand total received and sent fields in the address table, to handle extremely high value addresses in Dogecoin.
Add Dogecoin AuxPoW support

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.