Fixed: "Fatal error: spawn EMFILE" error for > 80 files.
Tweak to address failing compositor readImage() tests caused by async.mapLimit() inconsistencies.

Unlike async.map(), async.mapLimit():

* passes `undefined` instead of `null` when no error occurs
* apparently does not guarantee the output order matches the input order (?!?)

Added workarounds for both of these inconsistencies, so that readImages() returns the same result as before.
Merge pull request #7 from johnyanarella/compositor-process-spawn-limit

A "Fatal error: spawn EMFILE" error is thrown when a large number of files are specified.