Added subscripting support to FirebaseArray

[Vortec4800]

This is a pretty simple change, I added two methods to FirebaseArray which allows subscripting shorthand to be used.

http://nshipster.com/object-subscripting/

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please let us know the company's name.

[Vortec4800]

Signed CLA

[googlebot]

We found a Contributor License Agreement for you (the sender of this pull request), but were unable to find agreements for the commit author(s). If you authored these, maybe you used a different email address in the git commits than was used to sign the CLA (login here to double check)? If these were authored by someone else, then they will need to sign a CLA as well, and confirm that they're okay with these being contributed to Google.

[Vortec4800]

Added alternate email to Google account for CLA.

[googlebot]

CLAs look good, thanks!

[asciimike]

LGTM 🚢

Thanks for doing this. Next steps might be integrating this change into the code (where appropriate) to clean up all of the -objectAtIndex:index calls.