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.
> This should probably just be IPython.__version__. We aren't using version tuples in any other APIs.

But it's an API, it would require frontends to parse or version string, which we always know is sooo reliable. Or worse, people will start comparing strings and will be bitten by "10.0.0" < "9.0.0". And's it's not because we did things wrong elsewhere that we shouldn't do it right here.