Adds On Duplicate Key Update clause support

RFCR @nmonterroso @owend @SQLReview 

per http://dev.mysql.com/doc/refman/5.6/en/insert-on-duplicate.html

This will allow for fluent style upserts.

The first use case will be here https://github.com/Wikia/app/blob/dev/extensions/wikia/FacebookClient/FacebookMapModel.class.php#L270-270 for https://wikia-inc.atlassian.net/browse/UC-74
Actually, I think this class should take advantage of the [Set](https://github.com/Wikia/fluent-sql-php/blob/master/src/Clause/Set.php) class, creating set objects out of the key/values described in `$columns` at construct-time.