Make Placecomplete load Google Maps API itself, if it's not already loaded

Test Plan:
1. Start up server

    $ cd ~/khan/placecomplete
    $ python -m SimpleHTTPServer 9000

Confirm functionality
===
2. Load `localhost:9000/example.html`
3. Type "Pal" in Placecomplete input
4. Confirm "Palo Alto, CA" and other results appear.
5. Hit enter to select first result.
6. Confirm data loads and renders in page ("Event type", "Data value", etc.):
https://s3.amazonaws.com/uploads.hipchat.com/6574/98377/TkKoBUotmtI0FDo/upload.png
7. Confirm no JS errors in Chrome devtools console.
8. Click "x" in Placecomplete input to clear.
9. Repeat #7

Network tab
===
Confirming Maps API script loads later.

1. Before change:
https://s3.amazonaws.com/uploads.hipchat.com/6574/98377/PPkBqfFJWTNKL48/upload_png__1267×482_-3.png

2. After change:
https://s3.amazonaws.com/uploads.hipchat.com/6574/98377/rGNoHWDErewLEsu/upload_png__1255×379__and_HipChat_-_The_Stephanie_Chang-2.png

Reviewers: joel

Reviewed By: joel

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6107