Leading tabs detection in the source code

Leading tabs in the source code in our repo almost always mean that the file has not been formatted (i.e. never-formatted, or not formatted after adding new code). It's particularly easy to add leading tabs and overlook it in Eclipse.

No need to tell why unformatted code is bad.

Hence I've added a Grunt task checking for that. It only looks for leading tabs -- if there are mixed spaces and tabs, JSHint will report it.

**This task will be handy when importing the rest of our tests from CC to GH.**