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