Add function percentage

[hokaccha]

Before, percentage calculated by self.

@page: 960px;
@column: 300px;

.responsive(@count:1){
  @percent: (@column / @page * @count) * 100;
  width: ~'@{percent}%';
}

[role="main"] {
  .responsive(2);
}
[role="navigation"] {
  .responsive(1);
}

so added percetage function.

@page: 960px;
@column: 300px;

.responsive(@count:1){
  width: percentage(@column / @page * @count);
}

[role="main"] {
  .responsive(2);
}
[role="navigation"] {
  .responsive(1);
}

Like as Sass percentage.
http://sass-lang.com/docs/yardoc/Sass/Script/Functions.html#percentage-instance_method