Added a setSiteAliasUrls method to the API

This method works in a similar way to the addSiteAliasUrls method, but
it completely overwrites the URL list rather than merely adding to it.
Merge pull request #221 from crispygoth/SitesManager_setSiteAliasUrls

Added a setSiteAliasUrls method to the SitesManager API. Thanks for the Pull request!