Multiple Layers (baselayer XYZ)

Hi,
this PR comprises:
1.- Creation of the layers attribute in the leaflet element
2.- Support for multiple baselayers
3.- Support only XYZ baselayers (WMS, etc. later...)
4.- Example of usage
5.- Test unit of the new layer features

Please, take a look and comment this new functionality. Any help, hints, suggestions will be appreciated!!
Jaume.