README: add snippet for XTerm font integration
upgrade to Ruby 2.0 and newer ruby-git library
use bundler and document how to build these fonts
add Powerline symbols to create TamzenForPowerline
fix Powerline glyphs in screenshots by using XTerm

Imagemagick's convert(1) failed to render Powerline symbols (Unicode) no
matter what I tried, but XTerm already rendered them correctly so I took
advantage of that fact and am now producing the screenshots using XTerm.