Feature/geo hash filter

Hello, 

this is short summary what I have done to implement this feature:

1.

I've added new property to GeoPointMapping: GeoHashPrefix.
GeoPointMappingDescriptor has been changed as well.

2.
 
I've created new classes: GeoHashCellFilter, GeoHashCellFilterDescriptor and GeoHashCellFilterConverter 

3.
 
At the end I've prepared FilterContainer and FilterDescriptor to handle filter.


PS:
These classes contain tests for the issue: GeoHashCellFilterJson and GeoHashCellFilterTests  
@robertlyson Typically, we do not fully namespace our public API classes.  This should be changed to just `Nest`.