Allow blinken to be built as an uberjar

Add the :gen-class thing to govuk.blinken, so that `lein uberjar` will
generate a runnable jar file.

The intention is that we can then package up versioned .jars and upload them as [github releases](https://github.com/blog/1547-release-your-software).  This way we can have versioned artefacts, and also run blinken on machines which don't have leiningen installed.

To run an uberjar, just run `java -jar /path/to/blinken.jar /path/to/config.yml`.