Dumping boost::adjacency_list<> improves memory usage by a factor of
two, and also slightly improves performance in some cases.

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

This also modifies the behaviour of the graph_properties dict, which now
return the property values directly, instead of the map objects.

This fixes ticket #110

The string representation is now compressed, to avoid strings of huge
length, which makes pickle.dump() bork.

This allows them to be modified and passed forward in one expression.

After the removal of filtered vertices via Graph.purge_vertices(), the
values of known property maps are now kept consistent with the
unmodified graph.

This fixes ticket #97.

This also makes all Edge classes inherit from the same base class of the
same name.

This fixes tickets #94 and #95

This fixes a bug where the PropertyMap.fa attribute would not be set if
filtering is disabled.

This includes a spirit parser for the gml syntax, and a graph writer.

Additionaly, the general graph io code also been cleaned up.

This implements a general "blockmodel" generation / rewiring algorithm,
using Gibbs acceptence / rejection sampling (a.k.a Metropolis-Hastings).

This also implements some optimizations in the rewiring code, which
makes it more efficient on filtered graphs.