Added @import index resolution support

docs/import.md

@@ -18,6 +18,14 @@ will render to the literal css __@import__ shown below:
  When using __@import__ without the `.css` extension, it is assumed to be a Stylus sheet, for example `@import "mixins/border-radius"`.
 
  __@import__ works by iterating an array of directories, and seeing if this file lives in any of them, similar to node's `require.paths`. This array defaults to a single path which is derived from the `filename` option's dirname. So if your filename is `/tmp/testing/stylus/main.styl`, then import will look in `/tmp/testing/stylus/`.
+
+ __@import__ also supports index styles, meaning if you `@import blueprint`, it will resolve either `blueprint.styl` or `blueprint/index.styl`, useful for libraries to expose all of their features, but still allow a subset of the library to be imported. For example a common lib structure might be:
+
+    ./tablet
+      |-- index.styl 
+      |-- vendor.styl 
+      |-- buttons.styl 
+      |-- images.styl 
 
  In the example below we set the `paths` options to provide additional paths to Stylus. Within `./test.styl` we could then `@import "mixins/border-radius"` or `@import "border-radius"` since `./mixins` is exposed to Stylus.
 

lib/visitor/evaluator.js

@@ -517,7 +517,7 @@ Evaluator.prototype.visitImport = function(import){
     , root = this.root
     , i = this.rootIndex
     , stylus = require('../stylus')
-    , path = import.path
+    , path = name = import.path
     , relative = this.importPath;
 
   // Literal
@@ -527,6 +527,7 @@ Evaluator.prototype.visitImport = function(import){
   // Lookup
   if (relative) this.paths.push(relative);
   found = utils.lookup(path, this.paths, this.filename);
+  found = found || utils.lookup(name + '/index.styl', this.paths, this.filename);
   if (relative) this.paths.pop();
 
   // Throw if import failed