Use ESLint for Polymer

Modified version of #2600

- Ignores experimental and demo folders
- A few style tweaks
- Rename gulp task to `gulp lint`, and cause it to emit non-zero status
  for travis testing

Thanks to @fredj for the base PR

.eslintignore

@@ -0,0 +1,5 @@
+node_modules/*
+bower_components/*
+test/*
+src/**/demo/*
+src/**/experimental/*

.eslintrc

@@ -1,5 +1,9 @@
 {
   "extends": "eslint:recommended",
+  "rules": {
+    "no-cond-assign": [2, "except-parens"],
+    "no-console": 0
+  },
   "env": {
     "browser": true
   },

gulpfile.js

@@ -126,8 +126,9 @@ gulp.task('release', function(cb) {
   runseq('default', ['copy-bower-json', 'audit'], cb);
 });
 
-gulp.task('eslint', function() {
+gulp.task('lint', function() {
   return gulp.src('src/**/*.html')
     .pipe(eslint())
-    .pipe(eslint.format());
+    .pipe(eslint.format())
+    .pipe(eslint.failAfterError());
 });