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.

SET(LEMON_VERSION "@PACKAGE_VERSION@" CACHE STRING "LEMON version string.")