Add support for IE11.

The [previously submitted pull request](https://github.com/josh/useragent/pull/75) does not work – not for the version ```11.0.9600.16428``` of Internet Explorer.  Details on the new Internet Explorer User-Agent string format can be found [here](http://msdn.microsoft.com/en-us/library/bg182625.aspx) and [here](http://blogs.msdn.com/b/ieinternals/archive/2013/09/21/internet-explorer-11-user-agent-string-ua-string-sniffing-compatibility-with-gecko-webkit.aspx).

The [previously submitted pull request](https://github.com/josh/useragent/pull/75) expects to find the revision ```rv``` as the third element of the User-Agent application comment, but the revision is the second element.