Support for NATURAL JOIN (on top of JOIN USING)

[sneakertack]

Addresses #64 and #66.

@prust I've gone with your recommendation of still leaving the join criteria mandatory, except on cross join.

Here are the nuances of this commit:

• Calling .crossJoin('some_table') without any onCriteria is now the expected use case.
• However calling crossJoin with a criteria (e.g. .crossJoin('t2').on({'foo':'bar'})) will not fail; it will just silently discard ON/USING. You may or may not wish to change this.
• Ditto with the natural joins.
• Otherwise, any other join will throw an error if there is no on/using criteria.

// Criteria discard example.
sql.select().from('t1').crossJoin('t2').on({'foo':'bar'})
/* SELECT * FROM t1 CROSS JOIN t2 */

Note that this branch is based off the join-using branch.

[prust]

@sneakertack: Great work! I'm impressed that you added documentation & tests and that all of it (implementation, docs & tests) is in keeping with the styles and conventions of the project. Thank you!!

[prust]

@sneakertack: FYI, this has been pushed to npm as 1.2.0