[Filesystem] Fix and complete the API

Implement the spec.
Fix and pass the TCT tests.
Fix and complete the example.

BUG=XWALK-1071
BUG=XWALK-1827
I presume above action will generate `filesystem_api.js` under `filesystem/`, thus the source code tree will be polluted.

Could you generate to `INTERMEDIATE_DIR` instead? Hint, you might need change on `common/xwalk_js2c.gypi`.