Adds Model.count()

Method is available using chaining too, altought some options are
ignored (like offset, limit and order).
Fixes test-find-chain-limit table name
Adds Model.exists()
Adds instance.has*() for hasone and hasmany associations

Example:

```js
Person.hasOne("pet", Pet);

John.hasPet(Bobby, function (err, has) {
    console.log("has?", has);
});
```
Fixes instance.has*() hasMany association query, adds tests for instance.has*()