Insert citations into the doc (#184)

- Add general citations to modules.
- Add specific citations for max flow and min cost flow algorithms.
- Add citations for the supported LP and MIP solvers.
- Extend the main page.
- Replace inproceedings entries with the journal versions.
- Add a new bibtex entry about network simplex.
- Remove unwanted entries.

 #define LEMON_VERSION "@PROJECT_VERSION@"

 #cmakedefine LEMON_HAVE_LONG_LONG 1

 #cmakedefine LEMON_HAVE_LP 1

 #cmakedefine LEMON_HAVE_MIP 1

 #cmakedefine LEMON_HAVE_GLPK 1

 #cmakedefine LEMON_HAVE_CPLEX 1

 #cmakedefine LEMON_HAVE_CLP 1

 #cmakedefine LEMON_HAVE_CBC 1