Use absolute paths when including files.

The conditional in `index.php` to include the paths file is here to avoid `include` problems (somehow, the variables were giving empty values when using them in the `include` later. The file is called again in `common.php`.

`$installationPath` has the path to the root of the board.
`$libPath` is pretty much self explanatory.

I did some basic testing on this and it should work without problems.