Allow plugin use without invocation

Just a quick change to avoid having to invoke the function when using the default field names:

```javascript
var modella    = require('modella');
var timestamps = require('modella-timestamps');

var User = modella('User');

// Use the default field names:
User.use(timestamps);
// Or using the old syntax (it's backwards compatible):
User.use(timestamps()); 
// Or specify custom field names:
User.use(timestamps('born_at', 'changed_at'));
```

I've also updated the readme/history and bumped the version to 0.1.2. 

Let me know if I missed anything!

Cheers