Bug 283 import from ocid

Updates to cover initial review comments plus delta importing. Should be ready to start playing with real data now (hopefully?)
For the time values, I'd use 0 as the fallback, instead of -1.

```
>>> datetime.datetime.fromtimestamp(-1)
datetime.datetime(1970, 1, 1, 0, 59, 59)
>>> datetime.datetime.fromtimestamp(0)
datetime.datetime(1970, 1, 1, 1, 0)
```