Improve invalid record handling

- Use Yajl for JSON parsing
- Add ignore_invalid_record option to ignore invalid record and return OK to scribed

From this PR, `in_scribe` has 3 behaviours for invalid record.

- Return TRY_LATER to scribed(default for keeping existence behaviour)

```aconf
<source>
  type scribe
  # ...
</source>
```

- logging invalid record and return OK to scribed

```aconf
<source>
  type scribe
  # ...
  ignore_invalid_record
</source>
```

- No logging and return OK to scribed

```aconf
<source>
  type scribe
  # ...
  ignore_invalid_record
  log_level error
</source>
```