Require value for ttl when using Item as value

[tfwright]

Resolves #73

@sasa1977 thanks for considering these changes

I may have gone a little overboard on the README example changes. Let me know if you want me to scale that back :)

When considering addressing the use of the Item struct, it seemed like this line is conveying the point so I just changed it a bit to make the intention of the example more explicit.

Also, I normally wouldn't test code like this but let me know if you'd prefer to add something.

Finally, if you think this change does make sense, I wonder whether the value should be required as well?

[sasa1977]

Thanks! This looks fine to me. I agree that we should make value required too. I know it's strictly speaking not related to this PR, but I'm fine with sneaking it in :-)

[tfwright]

@sasa1977 snuck it in :)

[sasa1977]

Finally merged, sorry for the delay, was a bit busy. Thanks for the contribution!