Add support for Geometry Shader

[mrowqa]

It could be done like sf::RenderStates. You can keep method for one type of shader and write second method for all three types - if you don't want one of types you can simply pass empty string/stream (add sf::EmptyStream?). If one of shader's type won't be supported - you can simply fail loadFromXXX method - programmer should check which shaders are available before using them.

For checking which shaders are available you can simply modify existing method:
static bool isAvailable (Type shaderType);
Not bad idea is make it with binary flags.

[ghost]

It may be worth pointing out that the open-source mesa stack on Linux doesn't support geometry shaders, yet. So, it'd require proprietary drivers.

[eXpl0it3r]

Merged in 146f638