Simplify parsing of for loops

This also enforces that for-in does not have an initializer.
implement Array.of
remove source root for filename

Currently, this implementation is causing the path to double up - `some/file/url.js` becomes `some/file/url.js/some/file/url.js`.

The source root is optional, and only necessary if there are a number of source files that would benefit from having a common root.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1226

Closes #1226.
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
Destructuring does not require an object any more
0.0.57
Commit binaries for 0.0.57