Map Panel : Allow usage of term_stat facet

Hi, 

Here is a small contribution, i tried to better understand Kibana and ES, the i find myself hacking the the map panel... here is the result...

To summarize : 
- Provide a way to choose the stat to display into the map, not only the total number of hits
but also min/max/total/...
- Provide the necessary config, taken from term_stat panel
- Reload the data when we change the config