Add support for long interval names

The words `month`, `months`, `minute`, `minutes`, `second` and `seconds` are valid in PostgreSQL interval. These are now accepted in `parse_duration`.
Make greater interval units and weeks work, add more known aliases

* `millennium`, `millennia`, `millenniums`, `mil`, `mils` for a thousand years
* `century`, `centuries`, `cent`, `c` for a hundred years
* `decade`, `decades`, `dec`, `decs` for ten years
* `yr`, `yrs`, `y` shortcuts for years
* `week`, `weeks`, `w`
* also `d` for days
Merge pull request #3 from ralesk/master

Full interval unit names, more unit aliases, more units in general