implementations of RecordStoreFile and RecordStoreUtil

Here are implementations of RecordStoreFile and RecordStoreUtil natives, which some midlets use to store data in files.

There are more natives in RecordStore* classes, like in RecordStoreRegistry and RecordStoreSharedDBHeader, that still need implementing; but these seem to stand on their own more or less, and it seems better to land them and then iterate on those methods.

The implementations needed some fixes and enhancements to fs.js, so those are included, along with an initial set of fs.js unit tests.