This fixes ticket #84

This also belongs to an upcoming commit.

This modification belongs to an upcoming commit.

This changes stlightly the information passed to the auxiliary functions
provided by the user, so that more arbitrary structures can be
constructed more easily.

The condensation_graph() function now returns a property map which
contains the community values themselves for each vertex.

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

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

This fixes ticket:73

Images are now converted to png a posteriori for html output, but are
used as pdf for latex.

This fixes a compile error with gcc 4.4 and boost in debian squeeze.

This fixes a bug in the "correlated" mode in random_rewire().

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.