This fixes how paths are weighted in the absolute_trust algorithm.

This dumps the random-path approach, and implements a deterministic,
priority queue algorithm.

Random salt should be protected with "omp critical", and edge sampler
should check for the case when all edges have zero probability, and
sample uniformly.

Path sampling is done using different bias, and the edge sampling is now
done correctly. There is now only one parameter 'n_iter' which replaces
'min/max_iter' and 'epslon'.

The RNG object must be passed by _reference_ not by value, as is the
default! Otherwise the RNG state does not change...

The generators from boost::random seem to have a bug which causes them
to be biased. The generators from tr1::random seem to be in better
shape.

See: http://thread.gmane.org/gmane.comp.lib.boost.user/48006

Now paths are weighted according to their trust value until the last
hop, and paths are not counted twice.

Thins changes the graph filtering code slightly to wrap graph types with
GraphWrap, which automatically updates the edge index list when edges
are removed and added to the graph.

This also changes how graphs are passed to algorithms, which is now by
reference instead of pointer. (hence this touches lots of code, but
changes are trivial)

This adds the centrality module, which supports betweenness, eigentrust,
pagerank and absolute trust.