I ran into the same problem yesterday. The trick is, for full width spanning blocks, you just keep them out of the row/column structure, since row/column will always apply the default padding. Keep your footers and headers on their own, and use row/column inside them.
<header>
This will span the full width of the page
</header>
<div class="row">
<div class="twelve columns">
This text will flow within all typical padding and margins
</div>
</div>
<footer>
This will span the full width of the page
<div class="row">
<div class="twelve columns">
This text will flow within all typical padding and margins
</div>
</div>
</footer>