Add svgstore

Svgs are now automatically combined into a spritemap, fallbacks are
also automatically generated and a polyfill is included as well.
Several changes had to be made to allow this to happen:

- The development version of modernizr is no longer generated, but
  loaded from github with bower. This allows for scripts besides
  modernizr to be included in the <head> (because of the way we were
  replacing usemin's concatenated file in /.tmp with grunt modernizr).
- On a side note: plugins.js and main.js were removed. Since we're
  concatenating scripts it's no longer necessary to have these kinds
  of containers. You can just include scripts in the usemin block.
- svg4everybody.js is included as a polyfill (with bower).
- Added an example svg icon, as well as some example syntax (since it's
  pretty specific).
Merge branch 'svgstore' into develop