Add Boost Graph

[eddelbuettel]

RcppStream also needs boost/graph/graphviz.hpp

Looking at http://www.boost.org/doc/libs/1_57_0/libs/graph/doc/index.html:

The Boost Graph Library is a header-only library and does not need to be built to be used. The only exceptions are the GraphViz input parser and the GraphML parser.

so we can probably do as we've done in Boost Date.Time and include the headers only.

[eddelbuettel]

Moreover, this picks up an old thread on the mailing list so it may be of wider interest.