Add descriptions for contact form email (Name:, Email:, etc.)

  + prevent submit button outline on focus
Add 2x retina images and sprites

  + Used this mixin:
    https://github.com/AdamBrodzinski/Retina-Sprites-for-Compass
    to create 2x retina and normal sprites

  + remove unused images
Merge branch 'retina_sprites' into develop