Fixed: updated process.py

-Rewrote introductory notes to include Pyramid TIFF support and notes on various newly introduced options. Changed references of "process_jp2" to "process".

-Changed "PATH_TO_IMAGEMAGICK" and "PATH_TO_KDU_COMPRESS" global variables to command line options/default values for DivaConverter.

-Added fallbacks/graceful exits when convert or kdu_compress do not exist.

-Added graceful exit if file type is not tiff or jpeg.

-Added more detailed error messages for vipsCC failure.

-Changed VALID_EXTENSIONS to VALID_INPUT_EXTENSIONS for clarity.
Merge branch 'develop' of https://github.com/ddmal/diva.js into develop