Expose path on IDBFS, add missing node.js bits of path module

Doing anything beyond a simple test of the fs will require standard path operations.  We already bundle and use the node.js path module internally.  This patch exposes it on `IDBFS.Path` and adds the remaining bits that we didn't have before.

The `src/path.js` file is a copy of node.js' version without the Windows code, and with 3 changes (marked with `XXXidbfs` in the comments).