Add build script for bitcoind

bitcoind is part of the Bitcoin project, and it is used to run a server.
It is often (perhaps usually) needed without the context of the GUI, so
this is separate from bitcoin-qt.

There is a patch to fix homebrew with the Makefile, but it is currently
only available in HEAD. I'm going to versionize this as soon as the next
version comes out and has the new build data in the repository, which
should happen within the next few months.