Allow for more complex source and destination paths through middleware.

Support for a 'base' directory which is used to construct the path accessed via a request through through the middleware.
```javascript
app.use(stylus.middleware({
    src: __dirname + '/views'
  , dest: __dirname + '/public/stylesheets'
  , base: __dirname + '/public'
  , compile: compile
}));
.
.
app.use(express.static(__dirname + '/public'));
```

given a main.styl file located in /views/stylesheets; can reference it via:
```javascript
link(rel='stylesheet', href='/stylesheets/main.css')
```
These changes should not break any existing code.