Nanomsg included as source via git subtree

It's  looks like the prevailing pattern is for Node modules to either include the source code (i.e. node-expat) or to reference to a library that's located elsewhere on your system (pg, mysql). I would like to get rid of the need for a submodule (which works poorly with node-gyp in dev environments) but keep the separate history of nanomsg.

This PR attempts to use git subtree to solve both problems. Should be possible to lock versions of the code without munging the parent tree's history.

Thoughts ideas? /cc @nickdesaulniers @blowback