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This fixes #178.

This replaces a custom implementation of Ullmann's code.

This also implements direct manipulation of OpenMP settings (schedule
and number of threads) via python.

Use schedule(static, 100), to reduce overhead with small graphs in the
majority of cases.

The object may be globally reset via the seed_rng() function.

This works around a regression introduced in boost 1.48, relating to the
edge descriptors of reversed graphs.

See: https://svn.boost.org/trac/boost/ticket/6391

This allows for sampling of random subgraph isomorphisms.

Support for edge/vertex labelling is 99% done, but it is currently
impractical to compile with GCC. If GCC 4.5 doesn't do a better job,
then a workaround would be necessary.