Set default write concern for MongoDB

Set default write concern for MongoDB to safe to assure things are synced properly and to remove ugly warning message for newer versions of MongoDB:

    Please ensure that you set the default write concern for the database by setting
     one of the options
    
       w: (value of > -1 or the string 'majority'), where < 1 means
          no write acknowlegement
       journal: true/false, wait for flush to journal before acknowlegement
       fsync: true/false, wait for flush to file system before acknowlegement
    
    For backward compatibility safe is still supported and
     allows values of [true | false | {j:true} | {w:n, wtimeout:n} | {fsync:true}]
     the default value is false which means the driver receives does not
     return the information of the success/error of the insert/update/remove
    
     ex: new Db(new Server('localhost', 27017), {safe:false})
    
     http://www.mongodb.org/display/DOCS/getLastError+Command
    
    The default of no acknowlegement will change in the very near future
    
    This message will disappear when the default safe is set on the driver Db
Merge pull request #66 from mitar/patch-1

Set default write concern for MongoDB