Added Makefile and updated Dockerfile

The Dockerfile is now more like the weave one, with a scratch layer
underneath.

Makefile taken more or less from weave, too; except, it uses
environment variables to cross-compile the binary (which should
probably be done for weave as well)
Separated dockerising from code
Reformed Dockerfile and Makefile to account for moves
Moved main to top, libs to subpackage
Fix up Makefile and main.go for file moves
Merge remote-tracking branch 'upstream/master'

Conflicts:
	dns/Dockerfile