Pango

This branch uses Pango for font layout instead of the toy API that ships with Cairo.  I haven't started on `Cairo.Text`, yet, so this doesn't address #19.  It may fix #43; I'm not sure.

I also want to put together some examples that use non-Latin characters, BIDI, combining chars---all the nice stuff Pango should give us.  Pending figuring out what's wrong with my Fontconfig.

I'm seeking advice on the right way to interleave Pango and Cairo functions.  Pango returns in `IO`, rather than in `Cairo.Render`.  I hacked the code together with `unsafePerformIO`, which is obviously wrong.