Get client certs from vm, and set up the DOCKER_HOST with the real connection string

everything needed to make 

 `boot2docker init && boot2docker up && docker -H $(boot2docker socket 2>/dev/null) --tlsverify run hello-world` 

just work.

(requires https://github.com/boot2docker/boot2docker/pull/477 )

`--tlsverify` should disappear when https://github.com/docker/docker/pull/7530 or related PR merges into Docker 1.2.0.

I also needed a variation on #183, but called it `boot2docker socket` so 
Closes #183
Is GetHomeDir being exported?  If not, lower-case.  Also, starting functions with 'get' in Go is not idiomatic... but I know is used heavily in this project... so your call.  (Otherwise, it would just be `homeDir` or `HomeDir`.)