Document inferred name in Polymer()

Fixes #351. Also removed the tag-name argument from many code excerpts on this page.

Also clarified (I hope) section on adding properties and methods. 

Wording on both of these may be somewhat controversial, so @sjmiles, @sorvell, PTAL.
Yeah, I wasn't really sure what to do with them. I thought Scott wanted to highlight this more, but I'd be happy to revert these.