WIP 47: Add support for segment.io, initial event instrumentation

Instrumented segment.io events for:

* Identify on signup [email, account id]
* Signed Up
* Signed In
* Signed Out
* Searched For [query]
* Read Conversations Index
* Sent New Message [action]
* Had Message Send Problem

![image](https://f.cloud.github.com/assets/206973/1539017/e509e59e-4cfb-11e3-8761-255656fe4565.png)