Adding a refreshed date to sessions

[peteruithoven]

On the client we'd like to detect when it's a good moment to refresh the session. Currently we only have a issued date (which is set to the date of a login) and a expires date. This means that after a refresh (which don't currently update the issued) it's impossible to determine a good moment to call refresh.
Also see: micky2be/superlogin-client#59

I propose we add a refreshed field?

Updating the issued doesn't really make sense to me since the token / key and password remain the same?

[colinskow]

I like the idea. I will consider this PR next time I get around to making fixes.