Use image for bucket name with docker, if box is unset

This allows multiple machines using the image to share the cache
and avoids the following error:

    /Applications/Vagrant/embedded/lib/ruby/2.0.0/pathname.rb:389:in
    `initialize': no implicit conversion of nil into String (TypeError)
Check if image is set with docker provider

If build_dir is set instead for building images on demand, there's no way to
know what to set the bucket name to, except perhaps parsing it from Dockerfile.
Add notice about yum bucket and windows hosts (closes GH-117)
Merge branch 'use_image_for_bucket_name_with_docker' of https://github.com/jbarbuto/vagrant-cachier
v0.9.0