Return dictionary for vector objects.

Also, change parameter name for single coordinates.
Change coordinate paramter to latlng/s

Also, refactor parameter checks.
Change center parameter name since it is a reserved keyword in some environments
Add handling of all layers.

This commit adds all layers objects (vector objects such as polylines as well) to
the internal HashMap so that they can be accessed later.
Accessing is done via ids created from a layer's hashcode.

Layers added via "addLayer" are still referred to with their 'name'
attribute!
Add functionality to remove a layer using an identifier
Add functionality to update vector layers (polyline etc)
Update comments
Remove default doc
Update example app
Redraw all layers when a layer is created
Start TileDownloadLayers automatically when created
Clean up javadoc