Feature: Settings for Leaflet plugins, default map center and zoom level.

Hi guys, 

Awesome work with this package :o) Here are my additions to the app:

* ability to include Leaflet plugins easily, e.g. {% leaflet_js plugins="draw, markercluster" %}
* settings for default map center and zoom level - SPATIAL_EXTENT is great, but what if I want to limit the map to a country, and want to center the map on the capital :o)

I also updated the documentation for the new settings.

Tested with Django 1.4, Python 2.7, MacOS

Cheers,
Boris