Call mocha in node_modules instead of assuming it's in PATH

Review URL: https://github.com/google/traceur-compiler/pull/365

Closes #365.
Add `String.fromCodePoint` polyfill

http://mths.be/fromcodepoint

Related: #368.

Review URL: https://github.com/google/traceur-compiler/pull/369

Closes #369.
Add `String.prototype.codePointAt` polyfill

http://mths.be/codepointat

Note that I didn’t add my name to `AUTHORS` as the contribution guide requires, because I have already done so in the other PR: #369.

Review URL: https://github.com/google/traceur-compiler/pull/368

Closes #368.