add polyfills for Object.assign and Promise

[tpetr]

Adds polyfills for Object.assign() and Promise for older brosers. Longer term fix is to use Babel to add all necessary polyfills.

/cc @stevenschlansker #1046

[tpetr]

@ssalinas we should also probably release a 0.6.1 for folks that aren't ready to update their Mesos cluster. We can square that away separately (probably via branching off the Singularity-0.6.0 tag and then cherry-picking commits in).

[ssalinas]

Yeah, sounds good. I can do the git work for that when we are ready to release

[stevenschlansker]

Thanks guys, you're the best! FWIW I solved my issue locally: https://addons.palemoon.org/extensions/other/pm-promises/ but I'm sure someone else will run into this as well.