bindActivityOnClasses option added

This is based from issue [#111](https://github.com/HubSpot/pace/issues/111) I submitted. New option will get an array of class names that will be binded to pace's progress bar `{bindActivityOnClasses: ["square", "my-logo"]}`.

Pace will insert `.pace-bind` class to those elements hence will let you do anything with it in your theme.
```css
/* my spinning div */
.pace-bind {
  -webkit-animation: pace-spinner 400ms linear infinite;
  -moz-animation: pace-spinner 400ms linear infinite;
  -ms-animation: pace-spinner 400ms linear infinite;
  -o-animation: pace-spinner 400ms linear infinite;
  animation: pace-spinner 400ms linear infinite;
}
```
Feel free to change code naming conventions (can't think of any). Thanks