Add CSS3 calc function

[blackfalcon]

// Calc mixin requires two arguments, $attribute and $value
// $value2 $dimension and $operator are optional

// Common use case is
// block
//   +calc(padding, em(20))

// output CSS
// block {
//   padding: calc(100% - 1.66667em);
// }

// If different values are required for either top/bottom or left/right, the mixin will evaluate if the variable is set or not and if that values equal each other.

// Common use cases are
// block
//   +calc(padding, 0, em(20))
//   +calc(padding, em(20), 0)
//   +calc(padding, em(20), em(20))

// output CSS
// block {
//   padding: 0 calc(100% - 1.66667em);
//   padding: calc(100% - 1.66667em) 0;
//   padding: calc(100% - 1.66667em);
// }

// If you require adjustment to the default calc width you must either pass in the thrid argument to pass in the fourth or refer to the forth by keyword

// block
//   +calc(padding, 0, em(20), $dimension:50%)

// output CSS
// block {
//   padding: 0 calc(50% - 1.66667em);
// }

// The calc function can accept a complex series of values, in this case the $value argument of the mixin is capable of accpeting a string.

// block
//   +calc(width, '4px) / 2 + 3px', $dimension: 10%)

// output CSS
// block {
//   width: calc(10% - 4px) / 2 + 3px);
// }

@mixin calc($attribute, $value, $value2: false, $dimension: 100%, $operator: "-") {
  @if $value2 == false {
    #{$attribute}: calc(#{$dimension} #{$operator} #{$value});
    #{$attribute}: -webkit-calc(#{$dimension} #{$operator} #{$value});
  }
  @else if $value == 0 {
    #{$attribute}: 0 calc(#{$dimension} #{$operator} #{$value2});
    #{$attribute}: 0 -webkit-calc(#{$dimension} #{$operator} #{$value2});
  }
  @else if $value2 == 0 {
    #{$attribute}: calc(#{$dimension} #{$operator} #{$value}) $value2;
    #{$attribute}: -webkit-calc(#{$dimension} #{$operator} #{$value}) $value2;
  }
  @else if $value2 == $value {
    #{$attribute}: calc(#{$dimension} #{$operator} #{$value});
    #{$attribute}: -webkit-calc(#{$dimension} #{$operator} #{$value});
  }
  @else {
    #{$attribute}: calc(#{$dimension} #{$operator} #{$value}) calc(#{$dimension} #{$operator} #{$value2});
    #{$attribute}: -webkit-calc(#{$dimension} #{$operator} #{$value}) -webkit-calc(#{$dimension} #{$operator} #{$value2});
  }
}