Fix #150 - Path resolution is done everytime an fs change happens

Fix #150
Here's what I think you should do.  In sync-filesystem, do this:

```js
var pathToSync; // local variable in ctor
...
// Manage path resolution for sync path
Object.defineProperty(self, 'pathToSync', {
    get: function() { return pathToSync; },
    set: function(path) {
      if(path) {
        pathToSync = resolvePath(pathToSync, path);
      } else {
        pathToSync = null;
      }
});
```

Now in this code you can just do `fs.pathToSync = pathCache;` and it will figure it out internally in the fs' setter.  You'd also need to deal with the `null` case.

Code above is untested, but that's the idea.