Nrepl

The nrepl backend works:

* Evaluation is handled by nrepl. The interaction is captured in datomic. 
* *out* is sent to the browsers console.log (mostly as proof-of-concept)
* it (should be) possible to interrupt evaluations. More work is needed here. For starters it's not captured in datomic. There should also be better client side ui for interrupting evaluations. For example a spinner while we haven't received a `:status "done"` from the nrepl server (and a button to interrupt evaluation)

I also changed the command line interface as we have more options now. To use a running nrepl server:

    lein run '{:nrepl-uri "nrepl://localhost:<port>"}'

If you want session to start an nrepl server simply run `lein run`.