This fixes some strange bugs where deterministic position updates lead
to strange artifacts.

The object may be globally reset via the seed_rng() function.

This implements a graceful fallback in case GTK cannot be loaded, which
can (currently) happen simply by not having a environment DISPLAY
variable.

Images are now converted to png a posteriori for html output, but are
used as pdf for latex.

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

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.

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