added 'prefix' option to Table.prototype.star()

Added a prefix option to `star()`, e.g. `user.star({ prefix: 'user_' })`.

Example,

```javascript
var query = user.select(user.star(), userInfo.star({ prefix: 'info_' }))
  .from(user.join(userInfo).on(userInfo.userId.equals(user.id)));
```

From this query you would get the following fields in the result:

```
| id | name | email | info_id | info_address | info_phone |
```

Basically, it makes it the query much less verbose when you want everything from multiple tables and their column names potentially conflict. You won't have to spell everything out.