Schema True/False flag

This allows a flag to be set that will allow arbitrary data keys to be persisted in schemaless databases.

There are a few ways of defining it. If an adapter supports schemaless it can set it's default `schema` flag to false and this will allow anything to be persisted. This should be the default for the disk and memory adapters.

You can set it on a per model basis:

```javascript
module.exports = {
   schema: false,
   attributes: {}
}
```

Or you can set it globally in the adapters.js config:

```javascript
module.exports.adapters = {
    mongo: {
        schema: false
    }
}
```

It's already pushed to Waterline and I'll push it to the disk, memory and mongo adapters after this.