README: document existing cookie.maxAge property
Add new cookie.ephemeral option defaulting to false

This option ensures that the cookie is expired by the browser when
it closes. It used to be the default but this was changed in
a2b144ccf7399451c338130bcd72cfc193635614.
Ephemeral state can now be changed in setDuration

Changing the ephemeral state will cause a new cookie to be set in
the browser.
Merge pull request #57 from fmarier/ephemeral_cookies

Ephemeral cookies