Move and variable-ize forms and buttons.

I was having issues overriding the button styles and actually didn't want them at all. Also it felt weird that the form variables were in the forms file when all other variables are in the variables file. Since the button placeholder uses the bourbon button mixin, we should have a way to use the other button style options.

- Move button styles to new file.
- Move forms variables to variables file.
- Make the button style a variable.