Fix Class implementation, Fixed BUG, Added texs

Fixed table merge bug by changing implementation.

Made ns.instances[<class>] a weak table allowing garbage collection of
instances with no hanging references.

Changed implementation of slash command creation functionality to a
class-based system which allows classes to create top-level slash
commands (IE /<slash_name>) while getting self passed it.

Added default EH textures to the addon's scope to prevent inconsitencies
arising from users using an addon like "Aurora" which replaces default
blizzard textures.

OnUpdate handlers are now limited to only one interval per registered
function.