write_tsdb plugin

One more go around of #630 and #702 applied to master.

![docs](https://cloud.githubusercontent.com/assets/90042/3903019/6bfd9bcc-22c6-11e4-90f3-a116a17ae240.png)
We're using the `%.0f` format and `CDTIME_T_TO_DOUBLE()` elsewhere. Could you use this here, too, so rounding behavior is consistent between plugins?