build - losslessly compress PNGs

Use OptiPNG to losslessly compress PNGs after preparing the output
directory.
build - generate MD5 manifest of all files

Will be useful when updating S3 bucket only with changed files, and
then only invalidating changed files in Cloudfront.
upload - initial commit of upload script

After building this will upload newly modified files to S3 and then
only invalidate the modified files on Cloudfront. Moreover if there
are gzipped versions of a file available it will upload that instead
and set the appropriate Content-Type header.