convert to using dayjs

[jmsunseri]

moment js is too bloated for my company to use in our projects but I really wanted to use your library so I forked it to remove that dependency. I left moment as a dev dependency so that I could validate that my changes matched your changes. I had to modify two test results because they relied on an assumption that the person running the test had a positive UTC offset I hope you don't mind.

Not knowing if you are at all interested in this work I modified some of the information in the package.json. If you are actually interested in merging this then I will revert those changes and update the pull request.

Thanks,
Justin

[adamgibbons]

@jmsunseri Always happy to help ICS drop some weight - so thank you for opening this PR. I haven't reviewed it carefully yet but looking at it briefly, 1) I see only upsides to using Day.js; 2) I want to remove the positive offset assumption (it's a bad assumption!). I just need to double-check that this is has backwards compatibility w/ 2.x. So - yes - plz remove the package.json changes and I'll merge this in asap. Thanks again for making this a leaner library!

[jmsunseri]

I think that should do it.