Defaults

### What I did
- [ ] Set all variables to `!default`
- [ ] Sprinkled the variables throughout the rest file
- [ ] Did some light testing to make sure defaults are working and able to be overwritten.
- [ ] Created some additional variables for things like the font-size that's set on the `body` element in reset. And set the variable to 0.8125em (aka 13px)
- [ ] Added some default breakpoint variables. 

### How this might affect Upbase users
I didn't check to see if this would be a breaking change or not. I don't believe it is. However, it does change the order Upbase should be imported into a project. It now looks something like this:
  1. `@import base/variables` The variables file inside whatever project you're working on.  This should come before Upbase, as that's how the defa
  1. `@import base`
  1. `@import variables` This is the Upbase variables file
  1. `@import layout`
  1. `@import mixins`
  1. `@import reset `
  1. All other project specific files

I've also created a gist that could be copied into a project, containing the default values for variables. If we adopt these changes, should a file like this exist within Upbase or as something that can be copied and pasted into a project?