Redo the HTML representation; needs debugging

Puts HTML.Tag, toHTML, and toJS in the "htmljs" package.
Introduces toText for textareas and attributes.
Changes dynamic attribute representation.
Instances of HTML.Tag, HTML.CharRef, etc. are now "instanceof".

Once this code seems to work, we'll move packages around further.
- "htmljs" is the runtime; can even take out "tocode.js"
- "html-tools" (new package) takes over for "html".

Then, port the tests.
seems to work (not fully tested)
more tests and fixes

- moved tojs.js into spacebars

Some failing tests in `spacebars-tests` and `ui`

```
meteor test-packages spacebars htmljs html spacebars-tests ui
```
bug fixes
attribute compilation tests
getTag for unknown elements
pass tests
tests for details of new dyn attribute behavior
Merge branch 'shark-html2' into shark