Added Model nesting by using the 'inline' keyword

Here's another feature inspired from [mgo](http://labix.org/mgo). It allows beedb's reflection to marshall/unmarshall the nested structs as if its fields were at the same level as the main struct's fields.
```go
//example
var orm beedb.Model

type SQLModel struct {
	Id       int       `beedb:"PK" sql:"id"`
	Created  time.Time `sql:"created"`
	Modified time.Time `sql:"modified"`
}
type User struct {
	SQLModel `sql:",inline"`
	Name     string `sql:"name" tname:"fn_group"`
	Auth     int    `sql:"auth"`
}

func main() {
	// ...
	user := &User{}
	// ...
	orm.Where("id=?", 1).Find(user)
	fmt.Println(user.Id)
	fmt.Println(user.Created)
	fmt.Println(user.Name)
}
```