Add a -version command

Pending a process to assign major/minor version numbers, this is implemented by having the makefile embed the build date.  Addresses #10
This change also puts the version info and command-line arguments used to start the router at the top of its logfile, which will be useful when trying to understand what happened on someone else's system.
I think this should just say "build <date>".

And when builddate wasn't set, e.g. somebody built weave with 'go build' or some other way, this should return "unknown".