Declare npm package modules

package.json

@@ -6,8 +6,13 @@
   { "type": "MIT"
   , "url": "http://github.com/sstephenson/eco/raw/master/LICENSE"
   }]
-, "main": "./lib/eco"
-, "directories": {"lib": "./lib/eco"}
+, "modules":
+  { "index": "./lib/eco"
+  , "compiler": "./lib/eco/compiler"
+  , "preprocessor": "./lib/eco/preprocessor"
+  , "scanner": "./lib/eco/scanner"
+  , "util": "./lib/eco/util"
+  }
 , "repository":
   { "type": "git"
   , "url": "http://github.com/sstephenson/eco.git"