Buffer should convert to UTF8 not ASCII

I think [line 37](https://github.com/awslabs/lambda-refarch-streamprocessing/blob/master/ddb_eventprocessor.js#L37) should be rewritten as `payload = new Buffer(record.kinesis.data, 'base64').toString('utf8');`

I recently ran into a situation where I could not for the life of me figure out why I kept getting this mysterious `unexpected token error` in Lambda's Cloudwatch console. After a few hours of testing and debugging, I finally figured out that I was passing invalid JSON Control Characters. BUT, what was really happening was I was converting chinese characters to ASCII.
The standard should be UTF8 to support our global connectivity 😉 .

从 ddb_eventprocessor.js 的第 37 行开始，检查 Kinesis payload 在进行 JSON 处理之前是如何解码的。通过检查包含中文或其他非 ASCII 字符的 payload 是否得到保留且不再产生无效 JSON 控制字符错误，确认此更改。

由索引模型根据 Issue 内容生成。