Create method to select multiple zones

[fernaper]

Right now we can call the method select_zone() to draw a zone with his tags.
If you want to select more than a zone you need to call this method multiple times.
This is okay, but it is not perfect becouse the "intelligence" to position the tags don't ask the rest of the calls to that method.
Therefore, I think it is a good idea to create a new method called select_multiple_zone().
This method will try to posicionate correctly all the tags knowing the other zones.

Porbably we will need to add another library to do this: https://labix.org/python-constraint