Absolute time (t is now a double unix timestamp)

Closes #10.

[This](https://github.com/danielrichman/tawhiri/commit/632beee1f6a45c9e31f829edb26810d67d820140) is the actual commit.

We pass around timestamps between models since doing otherwise is slow, but I feel like the external interface should be datetimes—more Pythonic? That's why `solve(t, ...)` and `make_time_termination` take a `datetime` as its first argument (and immediately convert it to a timestamp).

I am (continuing to) use timezone-naïve datetime with values that must be UTC objects everywhere. This fact should be documented somewhere.