Fixing some issues with GeoJSON serialization

- Hardenning the FeatureJSON2 class (try/catching)
- Checking that a SRS can be guessed by OGRFeatureReader, or else
  falling back onto pure GeoTools implementation
- Using pure JSON lib to generate JSON instead of StringBuilders

This fix is related to PIGMA issue #75. Tests OK, manual tests on the 3
shapefiles provided by the customer:

* IAA_2009.zip
* infra_perigueux_a_jour_L3sud.zip
* SIEGE_EXPLOITATION_2008.zip