added synced_folder entries to Vagrantfile

Added two config.vm.synced_folder entries to enable a couple of useful
guest directories to be available from the host system.

This config does not create any directories on the host or guest system,
so before 'vagrant up' is run, the host directories (./sync/...) must be
created. Vagrant warns and exits if the local directories are not present.
enable synced folders only for virtualbox; gitignore sync folders

Updated synced folder config to be enabled only for virtualbox
provider. Added "sync/" entry to .gitignore.
added synced_folder entries to Vagrantfile

Added two config.vm.synced_folder entries to enable a couple of useful
guest directories to be available from the host system.

This config does not create any directories on the host or guest system,
so before 'vagrant up' is run, the host directories (./sync/...) must be
created. Vagrant warns and exits if the local directories are not present.
merging upstream/master
merged into upstream/master; updated README to mention VirtualBox syncing; fixed VirtualBox sync override config
added xvfb and firefox packages to tasks/dryad_deps.yml to enable headless Selenium tests in vm