Multiple changes: changes in build system, one workaround and one bug fix
- Optional compilation targets are now set in include/dai/dai_config.h instead of Makefile.ALL
- Workaround for bug in Boost Graph Library version 1.54 ("The graph may not contain an edge with negative weight") by not using Prim's minimum spanning tree algorithm anymore
- Small fix in swig/dai.i
[Julien Rebetez] added FactorGraph::printDot() to SWIG interface