[PROPOSE] API timestamp support

This pull request is to make it possible to specify the timestamp on API like:

```bash
curl -d 'number=10&timestamp=1400068922' 'http://localhost:5125/api/a/b/c'
```

although the default behavior does the rrdupdate with the current time. 

I especially want this feature to use GrowthForecast with [Norikra](http://norikra.github.io/). Norikra has a feature called [externally timed batch window](http://norikra.github.io/query.html) which sees the time field of event messages to aggregate. However, because current GrowthForecast does not support time field, I can not utilize the function of Norikra. 

Note that `rrdupdate` itself has a limitation that it does not accept `timestamp` which is newer than the last update time. I wrote detailed descriptions about how to implement this feature considering of the limitation at https://gist.github.com/sonots/3ddba0756d945c59ce5e (Japanese). This pull request implements the (1) way written on it.