Don't require the VBoxManage tool if we're not going to do path translation.

In the future, perhaps we should rename the VirtualBox interface to something
more general -- PathTranslator? -- then have a no-op implementation as well as a
VBoxManage implementation.
Trim leading slashes off of container names we get from `docker inspect`.

Do so immediately after getting the slash-prepended name from Docker, rather
than ad-hoc later on.

For reasons I don't fully understand, un-normalized container lookups (GET
/containers//foo_bar/json) were failing when made over a unix socket, rather
than a TCP connection.  Maybe go-dockerclient is wiring up the HTTP client in a
way that does path normalization over TCP but not unix sockets?
Rip out dependency on virtualbox

We still do path translation when DOCKER_HOST is tcp://, but always use the home
directory guessing logic rather than try to determine where vagrant has mounted
things.  @iansmith tells me that we were always taking this codepath before,
because vboxmanage doesn't report useful information on NFS shares.
Merge pull request #94 from kevinwallace/no_vagrant

Fixes for a non-Vagrant/vbox environment