Add support for Unicode escape sequences (\u{123})

Fixes #371
Partial sync up to changes in compat-table format.

Still a few cases which are incorrectly analyzed.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1303

Closes #1303.
Fix bug with too aggressive Unicode escape sequences

Fixes #1308
Merge branch 'master' of https://github.com/hzr/traceur-compiler