Store trace history

Stores the state of the entire trace in Redis after it runs. This state is then used by the Fuey web app to show a trace history. It currently retrieves the last 5000 states.

The state is stored as a `List` in Redis using the trace name (downcased) as the key.

```ruby
# example entry in redid for a trace named "IOS"
"{\"time\":\"20130909133059\",\"name\":\"IOS\",\"status\":\"executed\",\"status_message\":\"\",\"steps\":[{\"type\":\"Ping\",\"name\":\"Google\",\"status\":\"passed\",\"settings\":\"8.8.8.8\",\"status_message\":\"passed ping for 8.8.8.8\"},{\"type\":\"Ping\",\"name\":\"OpenDNS1\",\"status\":\"passed\",\"settings\":\"208.67.222.222\",\"status_message\":\"passed ping for 208.67.222.222\"},{\"type\":\"RFCPing\",\"name\":\"SAP\",\"status\":\"failed\",\"settings\":{\"ashost\":\"10.0.0.94\",\"sysnr\":\"00\",\"client\":\"400\",\"user\":\"weblogin\",\"lang\":\"EN\"},\"status_message\":\"RFCPing failed for 10.0.0.94. #<LoadError: cannot load such file -- nwsaprfc>\"}]}"
```

The history when viewed in Fuey looks like this:
![screen shot 2013-09-10 at 8 05 26 am](https://f.cloud.github.com/assets/56926/1114102/9c775ffa-1a10-11e3-876d-046f9df60dea.png)