Some improvements of graph_draw()

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

doc/conf.py

@@ -237,4 +237,3 @@ extlinks = {'ticket': ('http://projects.skewed.de/graph-tool/ticket/%s',
 # autodoc_default_flags = ['members']
 numpydoc_show_class_members = False
 autodoc_member_order = 'bysource'
-