add Date.toRFC3339

[namlook]

I wanted to pass a date to an html input and went into this issue: http://stackoverflow.com/questions/14212527/how-to-set-default-value-to-the-inputtype-date

Then I started to implement an helper to convert a Date into the RFC 3339 standard and I think your library is a good fit :)

I also exposed the function monthNumber as it was the first function I implemented for toRFC3339 and I think it may be helpful for other use.

[krisajenkins]

Thanks, this looks like a good addition. I'm going to make a couple of small tweaks, and then I'll merge it in and republish.

Oh, and I'll start a credits section in the README and add you. 👍

[krisajenkins]

Published as 25.7.0. Thanks! 👍