Initial support for cairo Surface and Context.

This change adds a new gotk3 package for Cairo bindings.  Currently,
there exists support for Surface (cairo_surface_t) and Context
(cairo_t).

Symbols used have been limited to Cairo 1.10 and not the latest 1.12
because I don't feel like adding yet another build tag to complicate
the build process.

This is a step towards completing #33.