Add vfs part

adds vfs part that replaces mdl_import part, adding multiple file
functionality (materials!) and now supporting file upload services like
dropbox or mediafire that obscure the filename of their downloads.
Usage:
Use lua function vfs.JSONCompress to create a JSON "archive" containing
all the files needed for a model (including materials). I'll add a pac
tool to make this easier later.
Upload this JSON "archive" somewhere
Add a vfs part as a child of a model part
Set the Forward Value of the vfs part to the full path of the mdl that
the parent model part should receive
Set the URL to where you uploaded the json "archive"
The model and its materials will be automatically downloaded, unpacked,
and the parent model will be updated to what you specified in the
forward value.

Caveats:
You can only have one vfs part per model (but that's fine since you can
package everything that model needs into the same archive)