URL encode the original image

This fix addresses #13 

Without url encoding thumbor will treat the generated url as malformed.

eg. 
This is invalid:
http://thumbor.example.com/{hash}=/fit-in/320x240/http://example.com/llamas.jpg

This is valid:
http://thumbor.example.com/{hash}=/fit-in/320x240/http%3A%2F%2Fexample.com%2Fllamas.jpg'

@harto @lwc