Re-support type="text/traceur" to enable global semantics for scripts

...tics for scripts

	Fixes #822

	The difference between scripts with type="module" and "text/traceur" is that the "text/traceur" script's globals will be global to all the other scripts, just like regular es5 script tags.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1033

Closes #1033, closes #822.
Rewrite BlockBindingTransfomer for better performance
Add missing copyright headers
Change parser to disallow import in Script by default

Also, update the placeholder parser to make parseStatement actually
start at ModuleItem (which in turn includes StatementListItem)

Fixes #1272