Add tagName to all overloaded DOM components

https://github.com/facebook/react/pull/873

Putting this PR here for discussion, if we should keep `tagName` in overloaded DOM components or not.

I chose to reference the initial tagName rather than redefine, compresses better and keeps them nicely in sync.

```
   raw     gz Compared to last run
     =      = build/JSXTransformer.js
  +167    +36 build/react-with-addons.js
  +108    +15 build/react-with-addons.min.js
  +167    +37 build/react.js
  +108    +18 build/react.min.js
```