session.created property

[GomZik]

Issue #4: Add created property with timestamp indicating the time when session was created

[asvetlov]

At first I very appreciate your contribution.
But I think we should use a bit another data structure.
Instead of pushing 'created' key into data dictionary we should create dictionary like:

    {'created': created_timestamp,
     'session': session_dict}

where current data moved into session_dict.
The benefits are:

1. we don't overlap session keys with our auxiliary ones. Say, user may want to have his own session['created'] value, why not?
2. I suspect created is not the only auxiliary session parameter. Say, we need to control session lifetime based on individual session info, not on system-wide value.

[GomZik]

Thanks for your review. I will rewrite that feature.

[GomZik]

Can you check my PR again, please?

[asvetlov]

Please add created info to the repr.

[GomZik]

Ok, i'l do it

[GomZik]

I made fixes from your comments

[asvetlov]

Thanks!