Improve Makefile.

This annoyed the hell out of me for a while, because my shoddy scripts
before would always force a recompilation of the target libraries
because it couldn't keep track of timestamps. This refactor makes it
such that if I type `make` twice, it doesn't recompile twice - just once.

It's not an end-all-be-all, granted, but it'll do.
Add Vagrantfile.

I recently switched my computer to a mbp; that's one of the reasons why
there's been a noticeable lack of progress lately. I still have my
other laptop, but... its battery life is terrible and it is literally
falling to pieces. </lifestory>

Anyway, this is good also, because it provides a consistent dev
environment to use. So use it.
go-duplex: update location to github.com/progrium/duplex

(long overdue)
Update README.md with new instructions and/or remarks.