Added a method to decompress a JPEG from a ByteTensor.  This allows one to store JPEG images in memory and then decompress them when needed.

changed to using jpeg_mem_src (since fmemopen isn't avaliable on Mac OS X and per @clementfarabet's suggestion).  Changed to THByteTensor_data(src) as per @soumith's suggestion.

switch to unsigned char ptr.
Merge pull request #30 from jonathantompson/decompress_jpeg

Added a method to decompress a JPEG from a ByteTensor.