now the user's tag in booklist page can link to the tag's page
the tag in book full view can link to its tag page
add SortHelper in memcache to speed up sorting; it can now sort by user's ratings
now it can sync to memcache while importing (done in merge_into_datastore() method)
now it can sync to memcache when editing a book's booklist info
it can sync to memcache while editing a specific book's information