adding nib support to stylus if nib module is installed

Ran into a very painful case where I use nib (`@import 'nib'`) in my stylesheets. In order for this to work, nib needs to be included when rendering with stylus. kss-node has no support for doing this out of the box. Heres a simple solution:

- In the stylus precompiler, try `require`-ing nib.
- If the user has nib installed and it was required, then include it when rendering styles. This way `@import 'nib'` lines will not fail.