Session data storage improvement

[akoncius]

Closes #98

[stangls]

This approach solves the issue of storing multiple values as session-data.
But I currently can not see in which way this closes #98.
The idea of #98 was to have type-safe access to session-data from the users perspective. For example, when storing an Int in the first request, I would like the framework to actually give me back an Int in the next request (instead of Any?).
In #98 I provided an example with the generic type T being returned when calling get on a SessionFetcher.

[akoncius]

ohhh, right. my bad. Will take a look

[rs3vans]

Any reason not to override the get and set operators here?