replace space (e.g. memory usage) with underscore

It is difficult to use (forwarding) terms including spaces to next fluentd.
So replace spaces with underscore like below:

```
2014-04-01 21:08:42 +1100 dstat.centos64: {"hostname":"centos64","dstat":{"memory_usage":{"used":"1968824320","free":"163844096","buff":"104062976","cach":"112762880"},"swap":{"used":"1442131968","free":"1146880"},"":{"":"1073733632"}}}
```

`memory usage` is changed to `memory_usage`.