Adds ability to dynamically find auto-increment id.

Previously, when persisting entities, we always start the
auto-incrementing id at 0, regardless of the entities already in the
table. This commit changes allows us to take the max value for the id column
of a table when finding the autogenerated id to persist.
Merge pull request #30 from karkum/Fix-finding-auto-gen-id

Adds ability to dynamically find auto-increment id.