Change variable-name and adding documentation

Change variable name $path to $font-file for declaring font-filename
because confusing with $font-path.
Adding inline documentation for sample files
Replace CSS reset with normalize

The old reset is customized normalize anyway, but without proper inline documentation.
Improving mixins and adding proper documentation

- Renaming 'dropdown' to 'absolute'
- Improving gradient mixin (change 'grad-dark' & 'grad-light' to single mixin 'gradient', support radial-gradient, better Sass conditional statement)