Fix parsing of `import {} from "A";` and `export {};`

https://code.google.com/p/esprima/issues/detail?id=615
Make tools/generate-test-fixture.js take a --module option for `sourceType="module"` mode

https://code.google.com/p/esprima/issues/detail?id=614
Merge remote-tracking branch 'ariya/harmony' into fb-harmony.jeffmerge