Fix some typos
Issue make before starting server under npm start.

Fixes #1151

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1168

Closes #1168, closes #1151.
Using npm git support for test-compat-table.

One time dev dependency rather than every clean.
Point to a fork of kangax repo for now.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1175

Closes #1175.
Enforce immutable bindings

This adds some core classes for visiting and building scope chains.

Then it uses that for FreeVariableChecker, VariableBinder and new
check that validates consts.

Fixes #18