implement default behavior for the partials_loader
lazily default partialsLoader to loader

Conflicts:
	src/Mustache/Engine.php
fix for when loader is not defined
fix test: a partialLoader should match the loader when neither are specified
Simplify `setPartials` nested ifs.
Move loader cascading into the loadPartial method.

This means the `partialsLoader` property is never set to the default loader, fixing things like `setPartials`, but template loading cascades as per the spec.

Add test coverage for partial cascading.