text: Title: Ability to set centre and zoom options on basic map
  Body: It would be good to be able to set the centre and zoom on maps (cf issue #80). An interface like:
  
  ``` python
  m = Map(zoom=8, centre=[-5.0, 5.0], centre_on_data=False)
  ```
  
  would work. The `centre_on_data` argument is useful to stop the default behaviour which recanters the map so it always bounds the data.
  