disk_free_space() should not be required

Was just trying this module out on a test site with PHP safe_mode enabled, and I noticed that the filesystem storage service uses the disk_free_space() function to determine how much free space is available. While this is certainly useful information, it triggers an error when disk_free_space() is unavailable.

I wrote a simple patch that addresses the issue; however, I haven't really looked at the rest of the code base, so I can't confirm that this doesn't break anything else …does this look like a good idea?

Alternatively, if the module is going to require functions like this that are disabled in certain security setups, it may be worth adding a note to the documentation about it.

Thanks!
Adam