Improve styling capability for [bw_navi]

[bobbingwide]

bw_navi uses bw_navi_s2eofn() to display the "s to e of n" at the start of navigation and
bw_navi_paginate_links to display the pagination. When content is being displayed in multiple columns then its quite hard to style the output nicely.
We need a better way to control the display.

Proposed solution

• In bw_navi_s2eofn change the span with class bw_se2ofn to a div
• In bw_navi_paginate_links add a div class="page-numbers pagination" around the output of paginate_links().