Fixes #88, suports DateTime in presence of ActiveSupport's Time.zone

I had to resort to using the `DateTime#to_time` and `Time#to_f`, which has clear-enough granularity for the years `(1900..2040)`, but loses resolution as we get further away from the epoch. The other solution I could thinkof was to go through `Time#iso8601`, but unsure if that is the way we wanted to go.

Fixes #88