[Geojson] Solution for collection and member "type"

[Sean Gillies]

I propose we make the "type" member for Features and FeatureCollections 
*optional*.

Geometries need to specify geometry type because multipoints and 
linestrings have otherwise identical representations. The "type" is 
needed here, and it is the "geometry type".

Features and FeatureCollections do not need a "type" member. Instances 
of these objects can be differentiated from objects of other kinds 
(including Geometries) entirely by

A) context: Geometries are contained by Features which are contained by 
FeatureCollections;
B) or by inspection: Features have a "geometry" member, 
FeatureCollections do not.

Applications may opt to add "type" members to objects for convenience, 
but it should not be mandatory. Comments?

Sean