Use double resolution logo for retina screens

Retina screens can display higher DPI images, and standard DPI images
(especially those containing text) can appear fuzzy.  This includes a
double size logo image, along with the CSS needed to display them.  To
make sure the 2x image matches the standard logo, a regenerated logo
image is included.  The responsive design for small screens has also
been adjusted to use the small logo instead of resizing the large logo.
don't force nav-list items to be too wide

The natural width of navlist items (with bootstrap 3) should be the parent's
width minus its padding.  Inheriting the width from the parent means the
padding isn't accounted for, making the width too large and breaking the
positioning of right aligned items.
reword subscribe link to be shorter and with better grammar

Shortening this link prevents it from wrapping.
fall back to first indexed module name in the absense of a NAME section

If there is no NAME section, we don't know what to call the current POD
page.  Using the name of the first module in the file (if possible) is
better than displaying nothing at all.
border and heading for toc, with inline toggle
hide toc with animation, maintaining width even when hidden
don't need sidebar toc toggle anymore
Merge pull request #1284 from haarg/toc-box

Border around table of contents with inline toggle button
Merge pull request #1279 from haarg/nav-width

Fix nav-list item width
Merge pull request #1278 from CPAN-API/haarg/retina-logo

Use double resolution logo for retina screens
Merge pull request #1280 from haarg/fallback-title

fall back to first indexed module name in the absense of a NAME section
fix busted image urls
add new logo images
use new urls for updated logo

Put in place new logo images as new files, and revert the old logo file back
to its original form.
fix 2x logo in responsive layout