Enable auxpow for Dogecoin

This PR is a rebase of @chizu's PR #446, against 1.8-dev.

Choices made:
- AuxPoW Chain id is set to *0x62*
- AuxPoW work will be accepted on testnet from block *158100*.

Please note that right now, there is a known issue that will force rebuilding the blockchain database, we're working on a fix for this.