Adds config option to specify a base disk image for persistence partition

Had issues with rebasing, rejecting PR #77 and created this one instead.

From #77:

Adds config option `basevmdk` to allow users to specify a base disk image to use for their persistence partition. For example, we have a `bootsync.sh` that's located on the persistence partition under `/var/lib/boot2docker/`. The way it currently works, we have to do something along the lines of:

```
$ rm VirtualBox\ VMs/boot2docker-vm/boot2docker-vm.vmdk
$ cp custom_b2d.vmdk VirtualBox\ VMs/boot2docker-vm/boot2docker-vm.vmdk
```
whenever we want to create a boot2docker vm from scratch. With this change, we're able to specify where that VMDK is in `$BOOT2DOCKER_PROFILE` and it is automatically copied in for us.