Remove non-core functionality. This lib should only handle annotating, while other stuff (like fading in and out a possible form) should be done outside this lib.

* Let overlay be fully transparent instead of semi-transparent
* Minor css tweaks to make it more robust when combined with other stylesheets
* Try to make textarea non-resizable
* Remove form-related code. Let it be handled outside this lib.
* Removed start() and commit() in favour of enable() and disable()
* Added toggle() that enables if disabled and vice versa
* Added event hooks for enabled, disabled, draw, drawn
* Removed unused settings
* Simplified example page
* Updated todo in readme