Fix #256; adds config to folder sync type

[ttres]

This change adds the option to choose which type of sync folders the user want to use. The allowed types are vboxfs (default for VirtualBox), nfs, smb and rsync. The default, in config.yaml, is vboxfs.

Fixes #256 .

[alvagante]

Very nice and useful, thank you.
I usually rely on the vbguest plugin to mount folders via vb additions, but give much more flexibility.

[ttres]

Good to know.
I use some boxes that do not have vb additions installed, and to install it on every boot is time consuming, so nfs is the way to mount synced folders.

[alvagante]

@ttres FYI, have had to push this 4461efc to fix errors with vboxfs