ALLJOYN-1683 Functions added to MsgArg for reading a writing AllJoyn structs

Functions added
   - alljoyn_msgarg_setstruct
   - alljoyn_msgarg_getnummembers
   - alljoyn_msgarg_getmember

Change-Id: I04eefa118ce57e5b8e8c26bb2e76a0c75e0a8e93