[boost] [Graph] New branch for graph library development (graph_devel)

I've started a new branch for further development of the graph library, graph_devel. At the moment it contains only a small amount of work (Brandes' algorithm for betweenness centrality and the Kamada-Kawai spring layout for undirected graphs), but it will be used for more aggressive development as well. Algorithms will be merged to mainline so long as we _aren't_ planning a release in the next few days, but any infrastructure changes will remain on the branch for a bit longer. I've only branched the boost/graph and libs/graph directories. Doug