Add omitna option to GeoDataFrame.to_json

When omitna is true, any properties that are NaN will not be written out.
This means that different features can have different numbers of properties,
which is okay for GeoJSON. Writing out NaN values is not valid JSON and causes
problems for some parsers.
ENH: Introduce method to handle NaNs in to_json()

This adds an 'na' option specifying how NaN values should be handled in
GeoDataFrame's to_json(). The default is 'null' which outputs JSON null.
The other options are 'drop' which removes any missing property, and 'keep'
which will output them as NaN.
Merge pull request #50 from jwass/geojson_omitna

Add omitna option to GeoDataFrame.to_json