Previously `notebook-mode` was an ordinary function that called
`notebook-minor-mode` to install `ein:notebook-mode-map`. Make
`ein:notebook-mode` a proper minor mode via `define-minor-mode`. This
has a few visible benefits primarily that `describe-mode` or `F1 m` will now
show all the keybindings, and mode-line will reflect.
As a consequence `ein:notebook-mode-hook` is no longer an explicit
`defcustom` (proper minor modes get it for free). This had been a
dangerous situation as the default hook containing critical functions could be overridden.
```
"http://localhost:8888"
"http://localhost:8888/"
"http://127.0.0.1:8888"
"http://127.0.0.1:8888/"
"8888"
8888
```
Ideally these should converge to the same thing. Since many hash
tables are keyed off `url-or-port`, forgetting to
normalize `url-or-port` with `ein:url` leads to missed cache hits and
general malaise. So we try to do that.
Address a FIXME: apply callbacks to `ein:notebook-list-login-and-open`.
Removed py3.5 from travis build matrix to reduce developer strain.
