Ticket #140 : support JSON map formats

I made further testing tonight, and did not see any further major issues, so I merging back the branch in to the main one :)


Current limitations :
* the backgroundcolor property is not currently exported by Tiled 0.9 (should be there in 0.9.1)
* TSX tileset file are not exported when using JSON, see https://github.com/bjorn/tiled/issues/381

Todo/known issues :
* fix the weird object issue when parsing tileset & layers (there is a workaround now that mask it)
* polygon/polyline support (currently disable)