Add boilerplate for jQuery code.

I have some doubts if you are going to merge this, as I see that https://github.com/h5bp/html5-boilerplate/issues/53 was closed.

However, I believe this is the type of things that should be in a boilerplate.

The main.js would now include:
1) Code to evade conflicts with other javascript libraries using the $() function.
2) Shortcut for $(document).ready(), used probably in 99% of websites that use jQuery.