Switch from `when` to `bluebird` for Promise implementation.

Bluebird is very fast.

Change-Id: I989720fc2ff24895c47fbdd52819148beefd4da4
Switch from `bluebird` to `es6-shim`/`prfun` for Promise implementation.

Use standard ES6 promises.

Change-Id: I78ffa583c77ea764152455454269151d13bb7f9d
Use `jpegtran` to purge EXIF data from jpegs.

XeLaTeX will still give images the wrong size when there is certain EXIF
information attached to the JPEG.  Use the `jpegtran` utility to clean
EXIF information from the JPEG before we give it to XeLaTeX.

Change-Id: I1c00dc80ff052d71371300c10b16d86e98fb02ab