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.

 prefix=@prefix@

 exec_prefix=@exec_prefix@

 libdir=@libdir@

 includedir=@includedir@

 Name: @PACKAGE_NAME@

 Description: Library for Efficient Modeling and Optimization in Networks

 Version: @PACKAGE_VERSION@

 Libs: -L${libdir} -lemon @GLPK_LIBS@ @CPLEX_LIBS@ @SOPLEX_LIBS@ @CLP_LIBS@ @CBC_LIBS@

 Cflags: -I${includedir}