Build bin/traceur.js in two explicit steps

Change make rules to build from master branch bin/traceur.js
First compile with previous branch bin/traceur.js.
Then use result to compile for self-hosting check.
Remove distclean rule; combine under make clean.
make test now depends on built traceur.js.

In most cases 'make test' is all you need.

Review URL: https://github.com/google/traceur-compiler/pull/493

Closes #493.
Merge branch 'master' into private-name-to-symbol
Code review fixes