Add ping library (forked from paulstuart/ping). Will add more to this,
and it should be held under github.com/docker/ eventually.

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)
Move ipallocator to erikh/ping

Docker-DCO-1.1-Signed-off-by: Erik Hollensbe <github@hollensbe.org> (github: erikh)