This introduces support for returning a generator in
subgraph_isomorphism(), so that all matches do not need to be kept on
memory simultaneously.

This fixes issue #255.

This fixes #189

This fixes a bug where equal edge descriptors would be considered
unequal. Additionally, now Vertex and Edges can be ordered (via >, <,
>=, <=).

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

This makes edge deletion O(1) if fast edge deletion is enabled for the
graph.

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

This fixes ticket #131, and includes a 'fast' O(k) option, which
changes the vertex index ordering.

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

An attempt no longer is made to reuse edge indexes in a contiguous
fashion.