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)
Can you pass the format as a string literal to *ssnprintf()* please? Using a variable as the format string immediately rings alarm bells, even though it's unjustified in this case.