Add HTTP User-Agent Parsing Filter

Adds a filter that can parse a user agent string in to structured data. This can be used for real-time analytics or other analysis. The parsing is based on the BrowserScope regexes YAML file from https://github.com/tobie/ua-parser. The parsing itself can be updated outside of updates to the filter itself by configuring the filter to read the regexes from a file.

One minor problem at the moment is that this filter depends on the user_agent_parser gem 2.0.0 which is currently not available from rubygems.org. Not sure how to work around this apart from waiting.

An example of what the filter does:

    Mozilla/5.0 (iPhone; CPU iPhone OS 6_0 like Mac OS X) AppleWebKit/536.26 (KHTML, like Gecko) Version/6.0 Mobile/10A403 Safari/8536.25

becomes

```json
{
  "ua": {
    "name": "Mobile Safari",
    "os": "iOS 6.0",
    "device":" iPhone",
    "major": "6", 
    "minor":"0"
  }
}