Added support for pin descriptions and the full size image

hey there, 

thanks for writing this useful gem :)

I added two changes:

1.  `parse_pin_image_url` will now return the full sized version of an image instead of the thumbnail that's returned in search results
2.  Added `parse_pin_description` which will return the description of the pin used when the pinner originally submitted the pin.  `parse_pin` calls this new method when fleshing out the pin hash