Add windows library path for sdl2 in setup.py

This pr does not affect the users of 1.9.0, because sdl2 will not be enabled by default.
Its purpose is to make sdl2 testing easier with kivy 1.9.0.

To start testing SDL2 with the current release, add the appropriate libaries to kivy/lib/sdl2 and set the correct environment variables.
A detailed wiki for testers will follow soon.
@tito that defines a search path for the sdl2 dlls and header files.
It is useful for portable packages. One only needs to add the dlls to:
```<portable_package>/kivy/kivy/lib/sdl2```
```setup.py``` will find them and use them, if the user wants to compile with sdl2.

In the future, when sdl2 becomes the default provider, we could ship the dlls with kivy, add them to the repo, but that is beyond the scope of this pr.