Add timestamp filter.

[wichert]

This adds a simple timestamp filter for places where you need a full timestamp instead of just a date.

[colinta]

what think you: https://gist.github.com/a83d6d39a3b981a487ad

I've brought in dateutil.parser.parse, moved string/int/float => datetime conversion into to_datetime()

see dc1650a for the entire commit

[wichert]

Looks good, although I'm wondering if you should use utcfromtimestamp instead.

[colinta]

i like it - also, i forgot to use to_datetime in timestamp(), that's been changed, too.