Add support for sending custom_data to KairosDB

I'm testing the [custom_data](https://github.com/proofpoint/kairosdb/tree/feature/custom_data) branch of KairosDB and wanted this library to work with it.

This only needs merged once the custom_data feature is officially included in the KairosDB client. I wanted to put it out here in case anyone else wanted to test this new feature using the kairosdb-client though. :)