Move Nav from genesis_before to genesis_before_header

[salcode]

The Genesis accessibility features add Skip links using the genesis_before_header. These skip links are:

• Skip to primary navigation
• Skip to content
• Skip to primary sidebar

These skip links are intended to come before the nav, however in Bootstrap Genesis they currently come after the nav. By moving the nav to genesis_before_header, this will correct this problem.

See #151

[salcode]

I don't love this change. This change means anything added to the genesis_before hook will appear before the nav.

I'm currently looking into why the skip links hook in to genesis_before_header instead of the earlier genesis_before, since anything added to the genesis_before hook will also appear before the skip links. e.g. http://www.carriedils.com/customize-genesis-move-header/