Use new template event after stylesheet links in head tag

Using this new event ensures the script is as close to the closing head tag as possible, and after any included CSS.

Since this relies on a [recently merged PR](https://github.com/phpbb/phpbb/pull/2756), the min. req. must now be raised to phpBB 3.1.0-RC3 (assuming that the next release will be RC3).

Closes #17