Enable the spaced-comment ESLint rule

Please see http://eslint.org/docs/rules/spaced-comment.

Note that the exceptions added for `line` comments are intended to still allow use of the old preprocessor without linting errors.
Also, I took the opportunity to improve the grammar slightly (w.r.t. capitalization and punctuation) for comments touched in the patch.

src/display/dom_utils.js

@@ -71,7 +71,7 @@ var CustomStyle = (function CustomStyleClosure() {
       }
     }
 
-    //if all fails then set to undefined
+    // If all fails then set to undefined.
     return (_cache[propName] = 'undefined');
   };