Added 0.5 to Default cards

[peval27]

I've fixed issue #138 so people who are estimating cna pick also 0.5 now.

[DavidBurela]

I'm not sure if it makes sense to add a 0.5 card to the deck. As you're just asking for something smaller than the smallest card.

Trivial things could be estimated at a 1 and everything else bumps up a size. If there is something smaller later, will it require a 0.25 card?

[peval27]

Other tools and physical card decks have 0.5. The idea is that if trivial things get 1 point, then it's more likely medium things will go to 5/8 which is normally a smell.
Adding 0.5 means you have the ability to use it, but can still skip it.