beamerthemetptng.sty: Add theme option authorinfoot

Enable the possibility to display the author name in the "institute field".

The declarations of the option authorinfoot is an adapted copy-paste of the textheighthack option.

Then, depending on the activation of the option, the institute field is modified in "institute in head/foot" For now, the following structure is used : <author> -- <institute>

We may want to let the user to decide the separating characters (the double dash is now in the sty ; user has to change the sty to change the separation)