Updated reame with DGS creation and use.

- Added a way to specify the number of ants in the DGS.
- Renamed TwoBridges.dgs in FourBridges.dgs (more exact :-))
- Renamed TwoSameBridges.dgs in FourSameBridges.dgs.
- Added TwoBridges.dgs (with really two bridges), which is the example
given in the readme.