Remove ObjectState (breaking change)

[Electron100]

ObjectState never really took on as large a role I had originally envisioned, while at the same time being quite clunky -- an extra field automatically injected into all model types. It had a single member saved, which tracks whether the object had previously been saved or not. Most of the time this doesn't matter though, especially since #117 changed save to upsert. I believe the only remaining case where it does matter is for automatic creation of primary keys, and those were operating in a semantic grey area where creating an unitialized primary key required setting it to -1 or similar.

This change does the following

1. Removes ObjectState and the auto-created state member for each model
2. Adds a new AutoPk<T> type which is used as the type for fields which represent automatic primary keys. This tracks whether the primary key has already been generated or not.

[jayvdb]

a few comments added, but overall this is awesome and lets get it in asap