Added support for Apache commons VFS2

Hi Richard Johnson, 

First of all, I'm not very used to doing pull requests. So forgive me if I made some obvious mistakes. 

I created a small fork of your libpst project because I'm writing a Apache Commons VFS2 file provide for PST files and I'm using your library to do so. In order to create a file provider, I needed support for VFS2 in the library itself. I'm aware this is another, possibly unwanted dependency to the library. Also I'm aware it may be of limited use outside the context of a file provider. 
Despite those two concerns I'd still like to give it a shot and send a pull request. 

With regards, 

Paul van Assen