API Added Site aware get_by_link

Added a multisite aware Site::get_by_link method as an alternative to
the default SiteTree::get_by_link method
Merge branch 'master' of git://github.com/sheadawson/silverstripe-multisites
FIX Removed incorrectly added local cache of variable
Merge pull request #26 from nyeholt/master

Fixed issue with too-aggressive caching causing errors with 404 publication