Mapfishapp - FeatureJSON2 Using pure JSON implementation

in the FeatureJSON2 class, some GeoJSON was generated "by hand", which
is error prone. This commit introduces the use of org.json java library
instead.
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
Merge branch 'pigma_issue_75-13.09' into 13.09