Document that the lua5.1 plugin loader is not thread-safe

https://bugzilla.gnome.org/show_bug.cgi?id=739619
Fix setting "peas-extension-type" in the lua5.1 plugin loader

This fix was already made to the other plugins loaders.
Make PeasEngine thread-safe

https://bugzilla.gnome.org/show_bug.cgi?id=739619
Add an extension test that uses multiple threads

This should act as a good stress test for checking that
libpeas is thread-safe. This also changes testing-utils to
be thread aware.

https://bugzilla.gnome.org/show_bug.cgi?id=739619
Cleanup PeasPluginLoader checks

Check that the vfuncs are implemented once, not each
time they are called.
Add nonglobal plugin loader support

This allows multiple threads to each have a PeasEngine
and be used without internal locking. It also allows using
lua5.1 plugins from multiple threads.

https://bugzilla.gnome.org/show_bug.cgi?id=739831
Cleanup lua5.1 test plugin

These properties were removed while developing the
plugin loader.