Add optional time zone when specifying page.date.

Update to documentation regarding the time zone used when specifying a post's date / time, as discussed in issue #1273.

Actually, in my testing, the date / time was taken as UTC when using Ruby v1.9.3, but was taken as local time when using Ruby v1.8.7. Given that Ruby v1.8.x is no longer maintained, I'm assuming I don't need to mention that in the documentation?