Replace date-ext with momentjs

[neocotic]

As the owner of date-ext, I can say that it's really no longer maintained. I did a recent update to tidy up the code, but that's about the extent of it. I suggest we move to momentjs which is much better maintained and has way more features and greater compatibility. However, this does mean that date format patterns will break as a result, so we'll need to inform users of this.

I strongly feel that the v2 release will required a post-update script to inform users of all changes within this release, highlighting the big and breaking changes like this.