Heatmap as a OpenLayers renderer.
Add weight as a APIProperty.

... and minor adjustments of documentation.
This code is not yet complete!
increases extend used by headmap with the radius.

This prevents the attenuation in areas near the edges of map.
Calculate max in `eraseFeatures` if renderer is not locked.
Use the weight stored in the item.
Set license.
User HTML5 doctype and adjusting indentation.
Allow more flexibility at max calculation.

In certain conditions it may be necessary to customize the automatic
calculation of the maximum.

Added to API properties:
 * `maxStart`
 * `updatableWeight`
Add more examples of OL renderer.
Merge pull request #47 from jorix/OL-Renderer

OL renderer for vectorial layers.