Added physfs support

I wrote some changes which I needed for my project. One is sandboxing all file operations, by using physfs (https://icculus.org/physfs/) which also brought nice features like reading from archives (like the quake-engine does) and the ability to produce stand-alone executables. I also dig out this UTF8 patch: http://forum.squirrel-lang.org/default.aspx?g=posts&m=3073 and applied it to newer code. 
I think the changes are better up in a repo than ending up as a patch file in a forum post. 

I just finished polishing on the PHYSFS part. With it you can use the macro SQPHYSFS to choose whether the normal file operations or physfs are used.

Once I have time I'll also cleanup the UTF8 support code and merge it.

PS: I don't know if merging against "squirrel3-genpurpose" makes sense, if it doesn't let me know.