Fix lots of compile warnings

This fixes loads of (obvious) warnings. 
Primarily code in the examples has been touched. Some warnings have been fixed within addons and OF core. 
Warnings in addon libraries (i.e. external code) have been ignored. Most of the obvious warnings (un/signed-compare and unused-variables) should be out of the way now, so we can concentrate on the non-trivial ones (e.g. some narrowing warnings).
Linux examples compile script has been adapted to ignore the recently added `osx*` examples.

Would be great to get some review!