Add multiple targeted font stacks

Focus on Helvetica Neue/Helvetica (default), Lucida Grande/Sans, DejaVu/Bitstream Vera/Verdana, Open Sans/Liberation Sans, and a sane high quality serif font stack.
Fix default theming hook

Make default theming hook more explicit
Closing #66 with both integration of #68 and using the same fonts. Users can add an optional class if they want these other convenient fonts. Also using a few fonts as backups for non-Mac users.
Merge branch 'master' of github.com:samuelclay/NewsBlur

* 'master' of github.com:samuelclay/NewsBlur:
  Changing 'Read It Later' in user-facing strings to 'Pocket' and updating URLs
  Re-wrote new user e-mail
  Changes to About page copy
  Remainder of FAQ re-writes
  Re-wrote answers for FAQ