Geojson exporter importer

This PR adds two drivers to export a spatial table to geojson format and import a geojson file to the database.
Supported [standard](http://geojson.org/geojson-spec.html) is 1.0.

Note:
1. Currently the geojson parse does not take into account bounding box and CRS. It will in the future. ;-)
2. The two drivers use the jackson library. So a new dependency has been added.
3. I also added a new function `ST_AsGeojson(the_geom)` that converts a geometry to its geojson representation.


Syntax:
```mysql

CALL GeojsonRead('/tmp/myfile.geojson', 'tableName'); -- Import
CALL GeojsonWrite('/tmp/myfile.geojson', 'tableName'); -- Export
```