Added ability to use #get with a query object

The #get model method now accepts an id or a query object. This
addresses the use-case where an error is desired when a row is not
found, but the query is on a column other than id.
Fixed documentation typo around skip/offset.
Added documentation for the afterFetch hook.
Merge pull request #18 from iancmyers/get-query

Let #get accept a query object and documentation