Added Triangle class and updated Intersector and ShapeRenderer

I added a Triangle class, which encapsulates a 2D Triangle defined by it's three points (A, B and C).
ShapeRenderer and Intersector now also support the Triangle class. In addition, ShapeRenderer now also supports directly called functions with object types Rectangle and Circle. (See source code below)

I'm kinda new to all this git and github, so please let me know if I did anything wrong(I tried my best). I would like to have some feedback :)

After looking at my commits, I should have written the class, which supports what. Can you change that afterwards?