Update documentation to reflect Dogecoin rather than Bitcoin

This replaces most references to Bitcoin in the documentation with Dogecoin. Need to reconcile differences between this and Dogecoin 1.6, to ensure we're not losing attribution for work done (especially Litecoin), and no errors are introduced as a side-effect.

OS X build instructions and Berkeley DB versions for UNIX build both should be checked very carefully, as I'm unsure on both.