Merge the login and open commands (open aliased to login). Add login
tests described in #352.
Attempt to improve user experience by synchronously executing
`ein:jupyter-server-start`. `ein:dev-prefer-deferred` custom variable
allows easy switch to compare sychronous versus old asynchronous behavior.
Use deferred and callbacks instead of `:sync t` for tkf requests which
is known to have issues. Query server attributes once on
notebooklist-open to avoid sequencing issue #176 (but allow Resync).
Under curl backend, a second request for the same "key" as a pending
request will abort the latter, which has resulted in a clobbered
curl-cookie-jar file, so merely warn and don't abort.
Fix#176
