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
Quoting with a single quote won't be sufficient for some cases.  Typically you'd want to use the quoting function supplied by the DB driver, but that's not available here.   However, it looks like our fluent SQL package provides a way to use parameter placeholders which is better for query caching anyway:
```php
$bk->append(" ?");
$bk->addParameter( $value );
```
See an example here: https://github.com/Wikia/fluent-sql-php/blob/on-dup-key-update/src/Clause/Values.php#L27