Added dncommon library for code sharing

The dncommon library is bare at the moment (only defines the current
version, but that probably isn't even needed since the version exists in
the project meta data) but functionality that is currently duplicated in
both dnserver and dnclient will be moved to dncommon because it's common
sense. When testing, dncommon must be installed to the local repository
using "lein install" before attempting to run dnserver or dnclient.