Adding cover and fit filters

![download](https://cloud.githubusercontent.com/assets/226553/3833313/1caf8b86-1da6-11e4-922a-d8a77a9914b0.jpeg)

http://localhost:8080/?l=a12345%3Acover()%3Bboxblur(10%25%2C10%25)%3Ba12345%3Around(100%25)%3Bpad(50%25)%3Bfit()%3B%241%3Aoverlay(%244)&w=600&h=338

`a12345:cover();boxblur(10%,10%);a12345:round(100%);pad(50%);fit();$1:overlay($4)`