Make bin/make-thumb.js work on batches

[zivester]

Currently make-thumb.js working on directories will kick off all converts at once for a set of files. Either make this serial or allow this to be done in batches. With any moderately sized set of inputs this will overload a cpu and lock up.

Possibly switch to just using promises, but this would require an upgrade to node 0.12. So possibly bump the minor version.