Add npm installation to homepage card.
Drop Bower icon and update styling for package managers
Merge branch 'site' of https://github.com/webcomponents/webcomponents.github.io into site

* 'site' of https://github.com/webcomponents/webcomponents.github.io:
  Add GA custom event to track webcomponents.js downloads
Improve margins for installation card.
Minor edits to Custom Elements doc.