Support for custom paths in Sass, Less and Coffee

Hi Tim,

I replaced hardcoded paths to stylesheets and scripts, like `public/stylesheets/sass`, with some fancy path detection. 

The detection depends on the corresponding gem being declared in `Gemfile`/`environment.rb`, then proceeds to look for files in a list of possible directories (like `app/stylesheets`, which seems pretty popular), then falls back to retrieving the path from the app itself (slow but sure, and only in rare cases).

Works with Sass, Less, and Coffeescript; I only tested it on Sass since that's what was available.

It's my first experience with vimscript - I tried to be nice. :)

Leonid.