Add b2d shellinit cmd

ignore the first commit, its from #224

make setting up the DOCKER_HOST, and optionally DOCKER_CERT_PATH as easy as '$(boot2docker shellinit)'

this will be used by the osx and windows start scripts to magically set the right env.
nit: For exported functions, it is common practice for Go that the comment immediately preceding the function starts with the name of the function.  This is done so that the `godoc` documentation reads naturally.  So this would be:
`// RequestCertsUsingSSH requests certs using SSH.  The assumption...`
Totally your call.  Just throwing it out there.