write_tsdb plugin

@pyr @fournier This is a re-roll of https://github.com/collectd/collectd/pull/630 after the botched merge.

Removed a couple bits of unused config code and added docs:
![docs](https://cloud.githubusercontent.com/assets/90042/3903019/6bfd9bcc-22c6-11e4-90f3-a116a17ae240.png)

I can forward port to master after this is merged.