Include README in npm package
Log value of a superClass when it's invalid

Specially useful as a better error message when the superClass is undefined

In docs, rename UniqueIdGenerator to UniqueIdentifierGenerator
Use template literals when logging an invalid superClass
Add to and export PlaceholderParser

- Makes the error reporting of PlaceholderParser more informative.
- Adds parseModule and parseScript template tags.
- Exports parseModule, parseScript, parseExpression, and parseStatement from
  the main Traceur module, as properties of the codegeneration export.
ensure traceur System is exported not the global
Exponentiation expression should be right associative

Fixes #1245
Refactor uses of codeUnit from ModuleVisitor to InternalLoader
Remove unused getModuleSymbolForModuleSpecifier()

Remove ModuleDescription in favor of ExportsList and adder function