Fix Graph-Tool Formula

The Graph-Tool formula was failing because the Boost formula does not enable Python support by default. Fixed by specifying it in the dependency.

Google SparseHash was listed as `:optional` before, but Graph-Tool actually *recommends* to depend on it. I changed it to `:recommended`. The Google SparseHash formula was failing on 1.9.2 according to Homebrew/homebrew#27652 (probably why it was `:optional` in the first place), but that has been [fixed](https://github.com/Homebrew/homebrew/commit/70872ed5cff55ef66b2fbd651265e09d58791315).

I also added a simple test according to the official quick start guide. Also, since it is possible to build without Cairo support, I added an extra option for that.