Hide navigation bar in front page, if breadcrumbs are activated

By default if one of the supported breadcrumbs plugins is activated, the `nav` element is showed also in the home page of the blog/site. An example: ![home-with-trail](https://cloud.githubusercontent.com/assets/6323899/2689345/9636d0cc-c2fc-11e3-8f70-426e11056e06.png)
I think that this should be prevented, because in a home page a breadcrumbs trail doesn't make much sense.
One could easily fix it putting this in the custom css: `.home nav { display:none; }`, because the `body` element has the `home` class only in the home page.
But I think that the theme should address this problem by default. Further, there is a php check in `header.php` that prevents a redundant function call when we are in the front page (credits to: http://mtekk.us/code/breadcrumb-navxt/faq/).

Tested with both the supported breadcrumbs plugins.

Regards,
Elvis

P.S. I didn't touch the minified version of `style.css`, in the root of the project, because I don't know how you like to handle it.