Sensible default library paths and ability to specify paths relative to Drupal installation

In 99% of sheetnode installations, the paths to required libraries will be precisely located as described in the setup instructions. Therefore it would be prudent to try using those paths when no configuration is set. The supplied patch makes it so.

And as a bonus, this change fixes bug #1867286 (Blank page when trying to save PHPExcel and TCPDF path settings). It also adds support for a <drupalroot> token in user-specified library paths--this is important for keeping self-contained Drupal installations environment-agnostic, and is exactly the sort of thing that would make a working dev site break when deploying to staging/production.