Variable helper path support

This patch makes the path of the box-sizing helper configurable by using a SASS variable.
The original CSS code has been copied as-is in a _scss_ file, moving only the path of the `boxsizing.htc` in a variable assigned with the `!global` modifier. When compiled, the result is actually the same original code, but when this library is included in a SASS file, the path of the helper can be configured overwriting the variable value, without making any modification to the library itself.

For example, when compiling the following scss file:

    // customize the helper path
    $box-sizing-polyfill-path: "/path/to/boxsizing.htc";

    // import bootstrap-ie7.scss
    @import "bootstrap-ie7";

the resulting CSS code will contain:

    ...
    behavior: url("/path/to/boxsizing.htc");
    ...