Changes Model to accept opts.id without default. No need, it's always defined
Adds settings object to orm

There's only 1 option available now. Example:

```js
console.log(orm.settings.get("properties.primary_key")); // "id"
orm.settings.get("properties.primary_key", "name");
```

This would force models created after this statement to use "name" as
primary key:

```js
Person.get("John", cb); // SELECT * FROM person WHERE name = 'John'
```
Fixes possible bug when removing instances where primary key is not called "id"
Adds test for primary key property name setting