Issue 736: Improved Alexa ID validation

Found in the WP forum that _ and - are allowed too
Merge branch 'master' of git://github.com/Yoast/wordpress-seo into Issue-736-2-improved-AlexaID-validation
Merge pull request #761 from jrfnl/Issue-736-2-improved-AlexaID-validation

Issue 736: Improved Alexa ID validation