Protect fatal errors caused by multiple inclusion.

Hi Gary, 

A small change which protects people who may pick this up and use it from fatal errors caused by duplicate class definitions.

Diff best viewed with ?w=0 - e.g. https://github.com/leewillis77/Gamajo-Template-Loader/commit/7939be314622681ef05a0390bfde816b946d34d6?w=0