GeoJSONFeatureCollection representing GeometryCollection entity

[admradius]

https://github.com/bryx-inc/GeoJSON.swift/blob/master/Pod/Classes/GeoJSONFeatureCollection.swift#L19
The type GeoJSONFeatureCollection is currently representing the entity https://tools.ietf.org/html/rfc7946#section-3.1.8 GeometryCollection as a FeatureCollection,
when FeatureCollection is defined separately as https://tools.ietf.org/html/rfc7946#section-3.3

Recommendations: Represent GeometryCollection and FeatureCollection as first class members of the EntityMap (typeMap) and create GeoJSONGeometryCollection to represent the GeometryCollection entity of the specification.

[harlanhaskins]

I'd really appreciate a PR for this! I'd be more than happy to merge it.