Unity Binding development work.

Removed the functions
    - alljoyn_msgarg_get_string_array
    - alljoyn_msgarg_get_objectpath_array
    - alljoyn_msgarg_get_signature_array

replaced with single function
    - alljoyn_msgarg_get_variant_array

Additional functions added for working with AllJoyn dictionary MsgArgs
    - alljoyn_msgarg_getkey
    - alljoyn_msgarg_getvalue
    - alljoyn_msgarg_setdictentry

The functions removed had been marked with the remark saying they only exist
for language binding development and could be removed at any time.

Many functions in the MsgArg.cc file were pre-fixed with 'extern AJ_API'
this prefix is only needed in the header file so it was removed.

Change-Id: I6040ddef56e72a43c341b19fe5c4712029f0caa8