Re-authentication occurs every 26.4 seconds
Re-authentication occurs every 26.4 seconds

Unit mixup's caused re-authentication to happen every 26.4 seconds,
in addition session length requested from Avanza is 60 days (86400 minutes).

The re-authentication timer was not cancelled properly upon manual call
to authenticate() (it needs to be cancelled with clearTimeout() not
clearInterval()).

Changed API code to enforce session length between 30 and 1440 minutes
as this is what Avanza offers in their own clients.

Requested session length is now derived from the same variable controlling
the re-authentication timer (so API user optionally can modify this).

(Fixes #9)

gustafg/avanza,fhqvst/avanza