Fix paths issue with modules in 'node_modules'.

Fixes #224, fixes #234.

BUG=https://github.com/google/traceur-compiler/issues/224
TEST=None (blocked by issue 221)

Review URL: https://github.com/google/traceur-compiler/issues/234
Merge pull request #236 from usrbincc/node_modules-paths--squashed

Fix paths issue with modules in 'node_modules'.