Rewrote SCSS to utilize placeholders & make class output optional

Several updates made to SCSS, mainly for the ability to turn off class output to use placeholder selectors for more semantic HTML

Updates
------------
- added placeholders selectors for every class (http://sass-lang.com/documentation/file.SASS_REFERENCE.html#placeholder_selectors_)
- added variable to decide if classes should be outputted ($fa-show-classes)
- made icon declarations in a list for looping reasons
- made a alias list as well
- made _icons.scss utilize loops of icon and alias lists for management ease

Usage
---------
```scss
//using placeholder selector
.my-class {
  @extend %fa;
  @extend %fa-star;
}
// same as: <i class="fa fa-star"></i>

//using with text inline
.my-button {
  @extend %fa-star;
  &:before {
    @extend %fa;
  }
}
//new: <a class="my-button">this is a button</a>
//old: <a><i class="fa fa-star"></i> this is a button</a>
```