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.

This introduces several simplifications and corrections to the graph
rewire algorithm, to guarantee unbiased sampling.

Now a move is outright rejected if it produces a
self-loop/parallel-edge, instead of retried. This also adds a
"non-sweep" mode, where edges are rewired randomly, possibly with
repetition.

The edge moves are now simplified to the target of the edges only,
since swaping sources is redundant.

The number of iterations can now be explicitly modified, so it is not
necessary to call the function more than once, and it is emphasized in
the documentation that only after sufficiently many iterations can the
graph be guaranteed to be fully mixed.

Newly added vertices or edges in filtered graphs now automatically
appear in them.

This better organizes the documentation of the Graph class and provides
other small fixes.

This allows pinning of individual vertices via the `pin` paramenter, as
well as string-valued color property maps, and some other minor
improvements.

This implements access to filtered graphs either with boolean-indexed
array or masked arrays, through the 'fa' and 'ma' attributes,
respectively.

For some reason sphinx looks for a graph_tool module inside graph_tool.
This commit defines such a thing.

This fixes a bug where filtered vertices would be incorrectly marked, if
the largest component index happend to be zero.

This fixes a bug where the filters where not correctly copied from the
base graph.

By using directly the C bindings to graphviz, we are able to return the
drawed graph as a string buffer in whatever format, which was not
possible with the SWIGified python interface.

Returning the drawed graph as a buffer is useful in doing animations,
among other things.

Now properties can be copied from vertices which are not identical
(e.g. from unfiltered to filtered graphs, etc). This fixes a problem
with graph copying and pruning, where the properties were not correclty
copied.

Now, statements like:

   prop[v].a = foo

are perfectly valid (if prop[v] is a vector type).

This works around a compilation bug in Apple GCC 4.2 + Macports.

This fixes an issue with group/ungroup_vector_property() and
random_layout().

This also includes proper test strings.