Modify the interface of MinCostArborescence + improvements (#267)

- Rename arborescenceValue() to arborescenceCost().
- Rename DefXyz template named paramaters to SetXyz.
- Rearrange public functions (for better doc).
- Doc improvements.
- Extend the test file with interface checking.

 prefix=@prefix@

 exec_prefix=@exec_prefix@

 libdir=@libdir@

 includedir=@includedir@

 Name: @PACKAGE_NAME@

 Description: Library of Efficient Models and Optimization in Networks

 Version: @PACKAGE_VERSION@

 Libs: -L${libdir} -lemon

 Cflags: -I${includedir}