This fixes ticket #110

The string representation is now compressed, to avoid strings of huge
length, which makes pickle.dump() bork.

This allows them to be modified and passed forward in one expression.

After the removal of filtered vertices via Graph.purge_vertices(), the
values of known property maps are now kept consistent with the
unmodified graph.

This fixes ticket #97.

This also makes all Edge classes inherit from the same base class of the
same name.

This fixes tickets #94 and #95

This fixes a bug where the PropertyMap.fa attribute would not be set if
filtering is disabled.

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

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

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.

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 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 the filters where not correctly copied from the
base graph.

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 also includes proper test strings.

This moves the documentation out of the constructor, and into the class
definition.

This allows easy creation of unfiltered copies of filtered graphs.

Now if a GraphView object is deleted, any property map created for it
reverts its ownership to the base Graph object. This eliminates the
problem of property maps created with transient GraphView objects.

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

This makes sphinx's autodoc slightly happier.