Have /api return the IPython version

closes #6647 

Mostly opening that to discus what we should have in here.

@millejoh would that suite you ?

```
$ curl http://localhost:8888/api
{"version": [3, 0, 0, "dev"]}
``` 

I'm reluctant on displaying more on a non-authenticated response, like os, commit....
and I believe at least this should be available on non-auth user so that at least they know how to authenticate.
> But it's an API, it would require frontends to parse or version string

Yup, as all version checks should. We store the IPython version in js in a string, too. Versions are generally stored in strings.