Fixed accessing DataSnapshot with multiple paths

[rhodgkins]

With this following data snapshot:

{
     foo: {
          bar: 'val'
     }
}

Calling snapshot.child('foo/bar').val() should be val - but as .child (and also .hasChild) just checks this._snapshotdata for the key without splitting it by / it returns null. Includes tests.

Also it would be useful to have this available so code that interacts with DataSnapshots directly (it might merge various snapshots into a single object, etc.) can be tested, so I've exported snapshot.js as DataSnapshot.

Finally I've also added node 8 to the travis test matrix - hope this is ok!

[rhodgkins]

I've fixed issue with accessing child data with number paths (for array indexes), including failing test case.

[dmurvihill]

Hi @rhodgkins, thanks for the contribution. As you may have guessed, this repo is no longer used (see #160), but work is continuing on my fork for now, and we will be making a release in early December. I've opened a new PR with these changes.

(Also, if you still happen to depend on firebase-mock, we are looking for maintainers).

[dmurvihill]

The sister PR has now been merged. Look for a release soon. Thanks again.