1. Neutralized unwanted button behaviour from elsewhere, 2. Only load font "slick" if needed, 

and 3. parameterize opacity

Slick is great. Thank you. This PR addresses three issues:

1. when I integrated `Slick` into my webpage, the buttons were showing odd behaviour. Since `Slick` is using the `<button>`-element for its buttons the slick buttons were impacted by `:hover` and `:focus` definitions from elsewhere in the CSS-styles. I neutralized such unwanted effects by explicitely neutralizing `:hover` and `:focus` effects. In addition I introduced a little `opacity`-effect when hovering buttons. This brought me to 

2. generalizing the opacity values. There are now three SASS-variables: `opacity-default`, `opacity-on-hover`, `opacity-not-active`. If you do not like a hovering-effect on the side buttons, just also set `opacity-on-hover: .75;`

3. If one wants to use a button font different from `$slick-font-family: "slick";` the `@font-face`--declaration is not necessary any longer. For instance, I took icons from [Font Awesome](http://fortawesome.github.io/Font-Awesome/), since I was using this font already. 

The following produces a rather nice alternative. 

    $slick-font-family: "FontAwesome";
    $slick-prev-character: '\f0a8' !default;
    $slick-next-character: '\f0a9' !default;
    $slick-dot-character: '\f111' !default;