Add Dogecoin support

These two commits add support for the Dogecoin network, and for AuxPoW blocks.

AuxPoW has not been isolated to the new Dogecoin module, as it may be of relevance for other coins added in future.

Further testing is ongoing, however I wanted to submit it as a PR now to get discussion started. Any feedback welcomed.

That's a lot easier from my point of view, had wondered why we were specifying it. Will just drop the version setting entirely, tomorrow.