message: Add get_amount()
diff: @@ -36,6 +36,7 @@ struct asset
  string symbol_name()const;
  int64_t precision()const;
  const symbol& get_symbol() const { return sym; }
  + share_type get_amount()const { return amount; }
  static asset from_string(const string& from);
  string to_string()const;
  