add precompiled binaries

sass.js

-var binding = require('./build/Release/binding')
+var binding;
+var fs = require('fs');
+try {
+  if (fs.realpathSync(__dirname + '/build')) {
+    // use the build version if it exists
+    binding = require(__dirname + '/build/Release/binding');
+  }
+} catch (e) {
+  // default to a precompiled binary if no build exists
+  binding = require(__dirname + '/precompiled/'+process.platform+'/binding');
+}
+if (binding === null) {
+  throw new Error('Cannot find appropriate binary library for node-sass');
+}
 
 exports.render = binding.render
 exports.middleware = require('./lib/middleware');