PNG logo instead of JPG

@SaraSchnadt mentioned that the logo looked blurry.

Try to correct this by generating a PNG from the SVG.
Open external links in new window/tab

Note that this is not my preferred behavior, but I can understand
why others like/expect it.

Give ``<a>`` elements a class of ``new-window`` to trigger this
behavior.
Spacing and alignment tweaks

I made the following visual tweaks based on feedback from
@SaraSchnadt:

* Make navbar thinner.
* Left justify navbar links.
* Right justify navbar icons.
* Left justify text in main column.
* Right justify text in sidebar.
* Vertically align default instruction text with main heading.
* Make blue color in map match logo (#258bd1)
* Increase logo font size to 40px and scale checkmark glyph
  relatively.

I made these structural changes to support the above changes:

* Make the main column 9 grid columns wide and the sidebar 3
  grid columns wide instead of (8/4).  The right aligned text
  looked weird with 4 columns of space.
* Use d3's ``selection.classed()`` instead of ``selection.attr()``
  to set sidebar classes.

I made the following additional changes that I encountered as I
looked at the code:

* Add page title to ``<title>``.
Merge branch 'master' into gh-pages