Refining the setting of site_tag when twitter:creator is present.

This is an attempt to close #935. I added a class method that
will take in the current value for site_tag and og_tags and
return a new value for site_tag if twitter:creator is set in
og_tags. If nothing is set, it returns the original value of
site_tag.
Merge branch 'master' into use-twitter-creator-for-twitter-site
Merge branch 'master' into use-twitter-creator-for-twitter-site
Moving call of new method to filter

Previously prioritize_creator_over_default_site was called directly
in twitter_cards_tags, but now it has been moved to the
jetpack_twitter_cards_site_tag filter so that it will be run after
site_tag when the filter is applied.
Changing priority in filter to allow overrides

prioritize_creator_over_default_site was previously set at a priority of -99, which would disallow other code to take its place.
In fact, it should be allowed to be overridden very liberally and the priority has been changed to 99.
Merge pull request #944 from enkrates/use-twitter-creator-for-twitter-site

Refining the setting of site_tag when twitter:creator is present.