Add timing code to the image decoding implementation 

Below changes have been implemented:
Build Servo, add timing code to the image decoding implementation in the net crate, rebuild Servo, and report numbers for different kinds of images (i.e. PNG, JPEG, GIF).

Build Servo and add code that reports (via println!) the time required to decode images into displayable pixels (image-related code)
Have you thought about using the `time!` macro for this? For examples on using it, see `components/layout/layout_task.rs`.