Add a "join" method to sets in order to support loading of many/belongs relations

[gi0baro]

Something like:

class Author(Model):
    name = Field()
    region = Field()
    has_many('books')

class Book(Model):
    name = Field()
    year = Field('int')
    belongs_to('author')

authors_with_books = db(Author.region == "Italy").join('books').select()
books_with_authors = db(Book.year >= 2015).join('author').select()