Switch from PgTimestamp to SystemTime.

[farodin91]

This would make calculations easier. It's also increase the support of different Database types. Diesel will start to support mysql in future.

[jimmycuadra]

As long as Diesel supports SystemTime (which it looks like it does) this sounds good to me!

[mujx]

One problem that came up in #137 is how to go from seconds (u64 etc) to SystemTime so that can be used in diesel queries. Maybe there is a workaround? We used the time crate (which is really missing from the std) for the conversions.

[farodin91]

Could be better to switch NaiveDateTime form chrono.

[mujx]

In general we should try to eliminate u64 and i64 where we can when it comes to timestamps.

[jimmycuadra]

chrono would be fine if the stdlib time APIs don't cut it.