Decouple nREPL server plugin from core server

[arrdem]

It should be possible to demote the nREPL development server from being a core part of Katamari to an optional development plugin.

Shouldn't be too hard - just move the CIDER/nREPL server into a server extension. For extra credit figure out how to make that extension a separate repo level module so that the stock Katamari uberjar doesn't have to bring all of CIDER along for the ride.