Only download boot2docker.iso one time

This works pretty well for me.  It might be nice to move to a single, globally-accessible constant for either `.docker` or the whole store directory since the individual drivers only have access to `d.storePath` (their unique location) right now.

```
$ # without "cache"
$ time machine create -d virtualbox dev0
INFO[0000] Downloading boot2docker.iso to /Users/nathanleclaire/.docker/images/boot2docker.iso...
INFO[0031] Creating SSH key...
INFO[0031] Creating VirtualBox VM...
INFO[0037] Starting VirtualBox VM...
INFO[0038] Waiting for VM to start...
INFO[0069] "dev0" has been created and is now the active machine. To point Docker at this machine, run: export DOCKER_HOST=$(machine url) DOCKER_AUTH=identity

real    1m9.102s
user    0m4.412s
sys     0m5.285s
$ time machine create -d virtualbox dev1
INFO[0000] Creating SSH key...
INFO[0000] Creating VirtualBox VM...
INFO[0006] Starting VirtualBox VM...
INFO[0006] Waiting for VM to start...
INFO[0037] "dev1" has been created and is now the active machine. To point Docker at this machine, run: export DOCKER_HOST=$(machine url) DOCKER_AUTH=identity

real    0m37.758s
user    0m3.854s
sys     0m4.967s
```

Shaves about 30 seconds off of local machine create time!

Signed-off-by: Nathan LeClaire <nathan.leclaire@gmail.com>