Continued Dogecoin support

Updated minimum work that can be required for a block to handle Dogecoin Digishield blocks.
Moved difficulty target calculation code into Bitcoin-Ruby.
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.
Return info on addresses only seen in 0conf txs
Speed up MemoryPool#remove_for_block
Fix PUT /transactions
Github link style
Change PUT /transactions to POST /transactions