Small fixes for KML write.

Be a good XML citizen and always write out schemaLocation.
Use serialize on ol.parser.XML so that we don't have to set xmlns manually.
RTree API improvements and more tests

This change reverts the RTree API back to the original one (i.e.
insert() instead of put(), search() instead of find()), and
creates a new searchReturningObject() method that returns an
object keyed by UIDs.

It also adds missing tests for type restricted search and
searchReturningObject().
Write support for ol.parser.GeoJSON

Also add some convenience with static read and write methods based on the
singleton.
Merge pull request #726 from bartvde/kml_serialize

Small fixes for KML write (r=@ahocevar)
Merge pull request #731 from ahocevar/rtree-api-tests

RTree API improvements and more tests. r=@twpayne
Merge pull request #732 from bartvde/geojson-write

Write support for ol.parser.GeoJSON (r=@tschaub)