Add support for converting WebTiledLayer and GeoRSSLayer from web map.
Extracted symbol parsing logic from RendererParser.

* Change how default symbols are set by providing default symbols instead of overriding the symbol creation functions.

* Expose individual symbol XML parsing functions.
Defer to SymbolParser.
Add GeoRSS layer symbol support.
Clean up.
Add 'webtiled' to valid-layer-types message.
Fix subdomain parsing typo.
Remove non-applicable display level parsing.
Remove unnecessary symbol to XML conversion.
Fix typo that prevented parsing PictureMarkerSymbols.
Add Base64 source property to <picturemarkersymbol/>.
Remove non-applicable service URL parsing.
Merge branch '#68/support-new-web-map-layers' into develop
Add support for Layer#showInLegend and ArcGISTiledMapServiceLayer#showInLegendHiddenLayers/ArcGISDynamicMapServiceLayer#showInLegendHiddenLayers.
Merge branch '#71/support-legend-specific-layer-properties' into develop