Move base64 encode of embedded images to Node.js

Trying to encode Base64 in ExtendScript was catastrophically slow for
large PNGs.  A 2.7MB file takes about 20 minutes.  Moved encoding to a
utility written in Node.JS where it's done in a fraction of a second
(uses an internal built-in).
Merge pull request #122 from adobe-photoshop/jp/svg-saveimage

Move base64 encode of embedded images to Node.js