Add #toGeoJSON to various layer types (#712)

[jfirebaugh]

Two shortcomings of this approach:

1. Features and FeatureCollections aren't round tripped. L.GeoJSON.geometryToLayer
   discards feature properties and I'm not seeing a clear way to keep those around for
   reserialization.
2. MultiPoints aren't round tripped. L.GeoJSON.geometryToLayer creates a FeatureGroup
   of Markers when it encounters a MultiPoint, but my implementation of
   FeatureGroup#toGeoJSON always creates a GeometryCollection.

@jfgirard's implementation in #712 contains a heuristic: if converting each layer
in a FeatureGroup to GeoJSON produces only Points, it returns a MultiPoint, otherwise
it returns a GeometryCollection. I don't like this solution, because it results in
the opposite problem: a GeometryCollection containing only Points can't be round tripped.

Instead I think Leaflet should have a dedicated MultiMarker class in the style of
MultiPolyline and MultiPolygon. But implementing that is tricky, because it needs to
support L.GeoJSON's pointToLayer option.

[ghuntley]

💓

[mourner]

Thanks John, very nice work! 👍 Let me think about the shortcomings a bit before I merge this...
Regarding MultiPolygon and MultiPolyline, I'm planning to make a different implementation for them in future as a part of vector layers refactoring (started here some time ago) — implement them as one path with multiple M points instead of a FeatureGroup.

[gamb]

+1

[mourner]

@jfirebaugh please rebase and update specs to Mocha style.

[jfirebaugh]

Done.

[mourner]

Reviewed this again, good to merge. Regarding Feature/FeatureCollection round-tripping — feature is saved as a feature property in the layer, so it can be used to form a Feature GeoJSON if it's present (extending it with an updated geometry).

[mourner]

@jfirebaugh see 7533a46 for initial work on roundtripping features.