upgrade style description API for v3

[incanus]

Now that style v3 is in as of mapbox/mapbox-gl-native#343 and we've got most other parts of the Cocoa updated as of d8b7e82...5938245, we need to tackle the methods around manipulating layer styles using native Cocoa types.

• MGLMapView

  - (NSDictionary *)getStyleDescriptionForLayer:(NSString *)layerName inClass:(NSString *)className;
  - (void)setStyleDescription:(NSDictionary *)styleDescription forLayer:(NSString *)layerName inClass:(NSString *)className;

• MGLStyleFunctionValue

• MGLTypes

Related: #31

[incanus]

Make that v4.

[incanus]

Related: #31.

[incanus]

Going to just roll this into #31 and use the specs going forward for dynamically creating strongly-typed APIs for style manipulation.