This modified several docstrings and introduces a quick-start guide.

Vertex/edge colors are properly plotted even when value range is zero
lenght. The behaviour of the 'fork' option was slightly modified.

graph_draw() won't work if gv is not available, and just a warning is
printed... This makes graph_tool.all work when gv is not available.

The seed option is now properly passed to graphviz.

Allow direct specification of vertex size, which accepts property maps,
and a couple of bug fixes.

This adds a graph_draw() functions which draws graphs in variety of
layouts and formats, and also to the screen, using the python graphviz
bindings.