SERVER-15500 New test for system.user operations

Closes #809

Signed-off-by: Benety Goh <benety@mongodb.com>
SERVER-15515 user updates in mixed version replSet

Closes #810

Signed-off-by: Benety Goh <benety@mongodb.com>
Merge branch 'v2.6' of github.com:mongodb/mongo into v2.6