Added reading of graph properties from graphml.
Merge remote-tracking branch 'myremote/master' into HEAD
Changed asserts to BOOST_CHECK in graphml_test.cpp
Comparison of doubles in graphml_test.cpp in now done with some tolerance to rounding errors.
Merge pull request #19 from josefcibulka/changes

Added graph property support to read_graphml

Well, it turns out there's already Boost.Test dependencies in Graph that I wasn't aware of.  Tested with Clang 6.0 on Darwin.