Added CSS3Pie .htc file and Library Comments

Since CSS3Pie requires a local .htc file, I added the ability to drop
in library comments in the header above the library code. Selecting the
CSS3Pie library simply drops in instructions on the location of the
library.

```
{
  "label": "CSS3Pie",
  "comment": "Add 'behavior: url(/js/vendor/PIE.htc);' to each CSS selector that you'd like to include PIE on."
}
```