Add basic host management with drivers
Add basic Digital Ocean driver
Add slightly better error for when vbm() fails
Remove a few modifyvm flags that aren't available on some VBox versions and shouldn't actually make a difference
Revert "Add the minor hack to get "docker hosts create DRIVER [OPTIONS] NAME" so "[OPTIONS]" can be driver-dependent"

This reverts commit 7e06571f2dddcc37550f4837c43561f457d9084e.

Signed-off-by: Andrew Page <admwiggin@gmail.com>
Add FIXME for driver-specific flags as discussed
Add basic active host support

Currently not possible to unset active host. Will perhaps fix itself
with a "default" host.
Improve error message for when hosts don't exist
Fix ".active" to not be +x
Rename "Unknown" state to "None"
Use flags to configure drivers
Make digital ocean image, region, size etc configurable
Add more logging when creating
Create host config before creating driver
Don't explode listing hosts if single host is broken
Rename drivers
Register drivers instead of hard coding
Don't check if boot2docker ISO has been downloaded
Make boot2docker URL configurable
Mount vbox share for OS X