Add !epoch_now yaml tag

Sometimes people get crazy and want to use Unix timestamps for
everything. So we added a special tag like the !now tag but this time it
returns the time in seconds since the epoch. It looks like:

```
birthday:
  fields:
    date_and_time: !epoch_now
```

Enjoi!
Add note to changelog about !epoch_now
Refactor to use timestamps with fraction

It is more powerful to default to timestamps with fractional seconds.
This adds that functionality.
Add test for offsets on epoch timestamps
Merge pull request #11 from uber/epoch-now

Add !epoch_now yaml tag