VMware host drivers for Fusion, vSphere and vCloud Air

Hello! this is a porting of what was bfirsh/docker#20 

I added support for identity auth all across the board and the previous notes are still valid:

- vSphere driver depends on vmware/govmomi/govc (must be in path) and works with vmware/govmomi@c8486300bfe19427e4f3226e3b3eac067717ef17 
- vCloud Air driver introduces a dependency on vmware/govcloudair@1df64d51cbbac795bdeeab8b4030cdf2e0471ccd (updated Godeps for this)

Thanks!
What about caching the boot2docker iso somewhere? something like ~/.docker/machine/iso_cache/boot2docker.iso and checking for a new release on `create` instead of just blindly downloading the ISO every time.