Adding the rotate filter

Added the rotate filter option that solves the issue https://github.com/thumbor/thumbor/issues/141

You will pass a number multiple of 90 and the image will be rotated according to this value, example: 
http://<thumbor-server>/unsafe/300x200/smart/filters:rotate(90)/s.glbimg.com/et/bb/f/original/2011/03/24/VN0JiwzmOw0b0lg.jpg