Removed Droid Serif and switched all references thereto to Open Sans

Removed Droid Serif from the font download package and switched all
references to it (or to serif fonts) to Open Sans/sans-serif fonts.
Added em-dashes that bracket the blog description in the header

A pair of em-dashes will appear on either side of the blog description
in the header. The content of the blog description itself doesn't
include the em-dashes, so it can be used elsewhere just as it's typed
up in the admin area.