Add standard date comparators

[BenJam]

I'm doing this at the moment so I might as well document it and submit a pull request answering this. Comments welcome but I'm going for something like:

before(date)

after(date)

between(date, date)

would be nice to add some sugar around notion of now, today, this week, this month, this year etc.

if anyone has an comments do let me know.

[thedersen]

For this to be included, it has to be i18n compatible.