GCE Driver.

This is a continuation of https://github.com/docker/machine/pull/7

I accidentally screwed up my remote branch, and then it got merged somehow.

I had a couple questions from that PR:

First, the GCE API uses oauth2 with refresh tokens. My current implementation caches tokens in driver.storePath, but this is obviously deleted when a machine is rm'ed, and only applies to a single machine. Would you be opposed to caching them one level up? So .docker/hosts/gce/ instead of .docker/hosts/gce//

Second, my test is more of an integration test and actually creates/rm's a machine. It's currently failing travis because it needs a real project ID to run against. Are you interested in having this kind of test? If so, how do you want me to block it from the travis run?

Signed-off-by: Dan Lorenc <lorenc.d@gmail.com>
is there a reason you don't inline there?
```
return &Driver{
  storePath: ...,
  sshKeyPath: ...,
  publicSSHKeyPath: ...,
}
```