Remove "polymer-" prefix

We shouldn't encourage people to use those prefixes:

http://webcomponents.github.io/articles/how-should-i-name-my-element/

According to Scott Miles on Polymer mailing list:

"The intent of the 'polymer-*' prefix in our stuff is to indicate 'elements
made by the polymer team', and not 'element made using polymer'.

Because part of the notion of Polymer is that users of an element don't
necessarily *need* to know it's made with Polymer (it should Just Work)
then don't feel like you need to identify it as such with 'polymer-*'"