Automatically update path options

[Compufreak345]

Adds support for http://leafletjs.com/reference.html#rectangle

Adds observers to the path options calling the new _updateStyle-method. If a feature is already defined, it's setStyle-method will be called with the updated options. This enables automatic updates for polygons, polylines and rectangles.

P.S.: Sorry for spamming pull requests, please tell me if it's not welcome or you want to collaborate in another way (all my merge requests are merged in my own fork, together with some (1) other features I'm not sure about). I am extensively using your library and implementing what I need on the fly - just want to give it back to your great project ;) I'm quite new to contributing on github, so I'm happy about any kind of feedback.