.jshintrc, code cleanup, bug fixed

Added `.jshintrc` with some default values.
Cleaned up `js` code accordingly to `jshint` preferences.

Fixed `Uncaught TypeError: Cannot read property 'parentNode' of null`: if user fires `.start()` before overlay entirely fades out there is the bug.

![intro-js-start-fire-bug](https://f.cloud.github.com/assets/556770/264719/9edc89be-8d98-11e2-97ef-dcad251e78d9.png)

It's a good idea to use debug versions of `js` and `css` in `example` until there are no make script in repository?