Enable CairoSharp to use native-cairo capabilities to write SVG output to streams

[MatthiasSchilder]

Added wrapping of cairo_svg_surface_create_for_stream (cairo API docs).

I've extended the WinFormDemo-application to show usage.

Clicking on "tosvgstream" will create a SvgSurface, which is bound to a .NET memorystream. After writing is finished, the stream is reset, read and its string content is printed to Console:

Please feel free to make any proposals for improvement or just do them yourself.

[MatthiasSchilder]

Added another commit which deals with the situation that the write_func delegate may be garbage-collected, even when the native code still needs it. This made the construction of a SvgSurface a little bit unaesthetic (initialization via constructor when you want to write to a file vs. via static method when you want to write to a stream), but I did not find another solution.