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.
There are a lot of gotchas to omitting the tag name, so I think it should go in its own section -- this section is already pretty complicated. (Should perhaps link to the earlier discussion, though and rephrase the following para to something like, "_tag-name_ can be omitted in some circumstances ...")