This fixes a leftover bug from the removal of the 'core' submodule.

Now GraphView.base points to the underlying, unfiltered, base graph,
even if "g" is another GraphView instance.

This makes sphinx's autodoc slightly happier.

This implements a GraphView class which allows for convenient,
independent filtering graphs.

Use Palatino as default font, and remove underline from links.

This adds a check for directed graphs in most functions of the flow
module, and renames the kolmogorov_max_flow to
koybov_kolmogorov_max_flow.

This also optimizes the graph augmentation in
koybov_kolmogorov_max_flow, where reversed edges are detected and used
if they exist.

Added more DOI links to references, and other cosmetic changes.

This also keeps compatibility with earlier versions of boost.

This fixes ticket 61:

http://projects.skewed.de/graph-tool/ticket/61

This module contains several search algorithms and a visitor interface.