Update addresses.py to add Dogecoin

Reference: https://github.com/dogecoin/dogecoin/blob/master/src/base58.h line 281

Dogecoin seems to be the darling of the reddit crowd at the moment. I'm planning on accepting it on one of my sites, so it would be useful if Coins was able to generate addresses with it.
Merge branch 'patch-1' of github.com:DeftNerd/coins into DeftNerd-patch-1
add support for dogecoin addresses
merge fix
fix name of DogecoinAddress
add in dogecoin unit tests