Lua bindings to librsvg

Using these bindings svg files can be drawn with lua and cairo. SVGs support real transparency (I tried it out using a transparent window) and can be scaled arbitrary.

Here's an exemplary lua snippet:

```lua
  if conky_window == nil then return end

  local cs = cairo_xlib_surface_create(conky_window.display,
                                       conky_window.drawable,
                                       conky_window.visual,
                                       conky_window.width,
                                       conky_window.height)
  local cr = cairo_create(cs)
  local rh = rsvg_create_handle_from_file(dir.."gpm-ac-adapter.svg")

  local rd = RsvgDimensionData:create()
  rsvg_handle_get_dimensions(rh, rd)

  w, h, em, ex = rd:get()

  print("width: "..w.." height: "..h.." em: "..em.." ex: "..ex)

  rsvg_handle_render_cairo(rh, cr)
  rsvg_destroy_handle(rh)
  cairo_destroy(cr)
  cairo_surface_destroy(cs)
```