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 allows pinning of individual vertices via the `pin` paramenter, as
well as string-valued color property maps, and some other minor
improvements.

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 filtered vertices would be incorrectly marked, if
the largest component index happend to be zero.

This fixes a bug where the filters where not correctly copied from the
base graph.

By using directly the C bindings to graphviz, we are able to return the
drawed graph as a string buffer in whatever format, which was not
possible with the SWIGified python interface.

Returning the drawed graph as a buffer is useful in doing animations,
among other things.

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 works around a compilation bug in Apple GCC 4.2 + Macports.

This fixes an issue with group/ungroup_vector_property() and
random_layout().

This also includes proper test strings.

Now, if 'm' is larger than the seed graph, it is set to the size of the
seed graph, and is increased progressively until it matches the desired
value. This removes the need to start with a random seed graph of the
appropriate size.

This also includes modifications which make the code more robust
against parameter choices which don't make much sense, e.g. lead to
negative probabilites.

This makes the calculation coherent with the definition given in the
documentation.