graph-tool: fix dep; flag changes; add test

Build failure fixed by requiring Python on Boost dependency.

Sparsehash dependency is recommended offically. Since the sparsehash
installation problem on 10.9.2 is now fixed (Homebrew/homebrew#27652),
it should be enabled by default.

Add option to drop Cairo dependency.

Add test based on the official quick start example
(http://graph-tool.skewed.de/static/doc/quickstart.html).

Closes #917.