Added submenu support

Including event handling per instance.
Updated all comments and added working namespace.

Pre-release version, should be stable, but running last tests.
Updated README.md

Changed readme to be in-line with the updates.
Changed .next() to .siblings()

More flexibility in mark-up this way.
Added classes to active link

Links that trigger submenus now get an 'is-active' class applied to them
(mainly for styling).
Merge pull request #1 from TheDutchCoder/submenu-support

Added submenu support and namespacing.