Adding a couple bits of missing API

In my project, I've come across a couple of bits of SFML API that are missing from the current DSFML

The first is the view property for RenderTarget--the C++ RenderTarget supports this, as do both of the CSFML subclasses, so I've added added it to the Interface. There are a couple other upstream methods that could also receive this treatment, and I'll look into adding those in the near future.

The second is the sf::sleep function. This seemed to be a pretty trivial addition. :)