Autoload classes

The current file layout may appear arbitrary to an outsider. For example, all the `P2P_Field_*` classes are in `admin/fields.php`, but P2P_Box is in it's own file.

Also, the classes are always loaded, whether they're needed or not.

The only downside is that `spl_autoload_register()` isn't guaranteed to exist in PHP < 5.3.