CSS based centering, removed prescriptive styling

The meat of the pull request is that I swapped out the JS based popup centering for an alternative implementation with CSS. I added default classnames to `qbMask` and `qbContent`, for those that are allergic to styling IDs, and removed and refactored some of the prescriptive styling that was originally included, so that users have less to override, if they so desire. (However, I resisted changing all of the ID and class names from your ludicrous and criminal camelCasing.) I also added an optional setting to blur the background when the popup element is displayed, specifically because it makes your eyes hurt.