absolute_trust() is now called trust_transitivity(), and the whole
trust matrix can now be efficiently calculated.

Vectors are used now instead of unordered_sets, which dramatically
improves performace.

This fixes a bug where edges were left unindexed.

This also maintains compatibility with older boost versions.

This fixes the expected values for several doctests, after
random_graph() re-factoring.

Now the degree sequence is verified with Erdös-Gallai inequalities, and
the edges are connected in a deterministic fashion. The edges then are
rewired with the random_rewire() function.

This includes also some internal refactoring, and bug fixes.

This extensively modifies the random_rewire() algorithm, so that only
either the source or the edge of each edge is rewired (not both, as
previously), and no parallel edges are created during the algorithm (if
desired).

The new version is much faster, and never gets stuck. However, more than
one run may be necessary in order to obtain a uniform shuffling.

This fixes a implicit directionality bug in random_rewire() with
undirected graphs.

This fixes a problem, where the underlying edge directionality would
cause correlations to arise when rewiring undirected graphs. Now each
edge is correctly considered in both possible orientations, which are
chosen randomly.

This implements a shortest_path() function which returns the shortest
path between two vertices.

This also modifies shortest_distances, such that a predecessor map is
optionally returned.

This fixes a crash when purging filtered vertices.

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.

This also cleans up some m4 boost macros, and includes other small
corrections to configure.ac.