Working on improving grid and visibility
Worked out some bugs and added table visibility's
Some small improvements

- Renamed most variables (it's not done yet)
- Removed some unquote()-functions
- Added a few lines of comments
Removed default! and changed breaking point variables

- Removed default! in _variables.scss
- Changed breaking point variables
Small bugfix

Small bugfix:
In _grid.scss the push and pull classes have a extra "-" in them.

Thanks to geoclicks (github)
Some small changes

- I changed .pull-gridname-1 to .gridname-pull-1. This is more consistent with the rest of the classes.
- Changed the variables because these caused problems. The name for $only-screen-grid (the mobile first grid) can't be used as well in $grid-names
- Removed a css selector "testing" that I used when I was testing the code.
- Added a visibility setting: $largest-breakingpoint-prefix. You can change the default "x" that is used for the largest visibility class in to something of choice.