refs #1638 : AlternateLogins model added

 - AlternateLogins model added (new framework-compatible type).
   variables : userid (int), provider (varchar), remoteid (varchar), data (array)
   methods:
    - get() : get login data from DBMS. (with pre-defined variables as qualifier)
    - set() : set login data to DBMS (every field should be filled)
    - remove() : remove login data from DBMS (with pre-defined variables as qualifier)
   NOTE that there is no routine to validate variables' types and values.