Updated Core Icons article for styling changes:

  fill => color
  size => width,height

Also added examples for styling icons inside other components,
edited for style, and made headings more consistent.
Updated to address comments.
Merge pull request #575 from arthurevans/icon-style

Updated Core Icons article for styling changes:
Make sure docs build under jekyll 2.2. Just use raw commands
Merge branch 'master' of https://github.com/Polymer/docs