ENH: make front page icons arrange better on narrow displays

The "Blog" icon on the front page often went below the right hand menu.
This commit makes them Bootstrap thumbnails, so that when the display is
too narrow, they rearrange to two rows instead.
STY: make front page navigation icons smaller to keep them on one row.

One row for reasonable browser screen width that is.
Merge pull request #1 from rgommers/pull-45

STY: make front page navigation icons smaller to keep them on one row.
Merge pull request #45 from pv/icon-thumbs

ENH: make front page icons arrange better on narrow displays