Add vagrant-bindfs for custom NFS permissions

This PR stems out of #82.

**Notes about options.** I added the options `u: 'vagrant' and g: 'www-data'` to override the `vagrant-bindfs` default of 'vagrant' for each. This frees the `wordpress-sites` role from having to perform a `chown` to `vagrant:www-data`. 

The [bindfs man page](http://bindfs.org/docs/bindfs.1.html) indicates that `-u` and `-g` cause `chown` and `chgrp` on the mounted filesystem to always fail. So, I assume anyone customizing their playbooks to `chown` to something other than `vagrant:www-data` will see a failure warning. In that case, the `--chown-ignore` and `--chgrp-ignore` options might be of use.

**Other Notes**
- I tested on OSX only. I'm not aware of any reason it wouldn't work on other NFS-compatible hosts.
- I omitted warnings for missing `vagrant-bindfs` plugin, a la `if !Vagrant.has_plugin? 'vagrant-binfs' ...`
- I didn't try anything with `mount_options: ["dmode=775,fmode=664"]`