Added feature grouping and layer clipping functionality.

* Features are now grouped into GeometryCollections, grouped by the
reuslt of the unique() function.
* Added SVG clipping, so that features overlapping multiple tiles with
a pixel overflow can have their outlines hidden.
* Updated README: added contributor and added better documentation.
Updated README formatting.
Added demo link to gist.
Fixed link in README.
Update README.md
Cleaned up code and added better documentation. Disabled mouse events on hidden tile rectangles.
Merge branch 'clipping-and-merging' of https://github.com/glenrobertson/leaflet-tilelayer-geojson into clipping-and-merging
Merge pull request #9 from glenrobertson/clipping-and-merging

Clipping and merging