Added ISpriteBatch, Texture2D.FromByteArray, faster RenderTarget2D.GetDa...

1) RenderTarget2D.GetData:  I updated this method to GL.ReadPixels if it is the currently bound RenderTarget.  This is immensely faster

2) Texture2D.FromByteArray:  This method allows for taking image byte data (gotten through Steam) and putting it into a Texture

3) MonoGame.cfg:  Allows you to make a config file to set the minimum OpenGL version and any required OpenGL extensions

4) ISpriteBatch:  This was added to allow you to write your own SpriteBatch but still utilize the DrawInto functionality of SpriteFont.  This is purely XNA extension and retains all the old behavior.