SMCalloutView contentView property setting implementation for RMMapLayer

Implements setting ```calloutContentView``` in ```RMMapLayer``` in order to use it in callout view the same way as ```rightAccessoryView``` or ```leftAccessoryView```.
As SMCalloutView's commentary on this property says about ```contentView``` property:
" If this is set, title/subtitle/titleView/subtitleView are all ignored. ".