Make pretty print an option of the ParseTreeWriter.

Make the tree writer take an option object containing a pretty print flag instead of hard coding to always use pretty printing.

Also, do not append a new line to the end o printing trees. We still append new lines after statements.

Review URL: https://github.com/google/traceur-compiler/pull/666

Closes #666.
Duplicate imports should be reported as errors

Fixes #648

Review URL: https://github.com/google/traceur-compiler/pull/667

Closes #667, closes #648.
Remove export symbol etc

Clean up our internal Symbols. The only one left after this is our ModuleSymbol.

Review URL: https://github.com/google/traceur-compiler/pull/668

Closes #668.
Extract --referrer prefix from package.json version.

node build/printReversion.js prints semver patch + 1 number.
node build/writeReversion.js updates packages.json version and
  devDependency version.
Require traceur node_module as devDependency.
Respond to review
respond to review
Don't impact  Makefile in this patch