[Ready] Changes centering calculations for modals

[p-shannon]

Fixes #7 and elementary/website#1255

I don't know how to test this so please tell me how to do so, or test it yourself and confirm the proposed changes are sufficient. Thank you!

[lewisgoddard]

To test you should be able to just open the index.html file in the repository and click any modal button there.

I think we're going to have to expressly include a container though, styling the modals normal parent will probably break some other styling. It's probably best if we have a container that is explicitly set as 100% width and 100% height to center in.

[p-shannon]

Alright, tested it and everything and it's good. Let me know if anything else needs doing (differently).

[lewisgoddard]

I'm closing this in favour of #13 where I've done basically the same thing by moving the existing overlay around and using that for positioning instead of creating a whole new element. Thanks for the help!

[p-shannon]

No problem. I actually tried to move it to the existing overlay, but I couldn't figure out how to resolve the resulting opacity issue. I'll look at your implementation to see how it was done.

[lewisgoddard]

I passed the opacity into an rgba background so that it wouldn't affect children like an actual opacity style would.

[p-shannon]

Yep, just saw the code now. I thought about that, but then I realized I'd have to figure out how exactly you handled the fade out. I also see you refactored the relevant bits significantly, even changing the overlay from a class to an ID, which isn't something I could have done.

[lewisgoddard]

Yeah, I ended up just breaking the fadeOut in favor of CSS Transitions. jQuery was getting some pretty poor framerates over on elementary.