Add optional balloon to Google Maps block

Let's allow users to add a balloon to the Google Maps block.

## Before - options ##
![pre-options](https://cloud.githubusercontent.com/assets/928116/2600414/cd296042-bafd-11e3-96ec-f00e7cd9f53b.png)

## Before - view ##
![pre-view](https://cloud.githubusercontent.com/assets/928116/2600422/fd2d8962-bafd-11e3-90ac-8556ec933ec2.png)

## After - options ##
![post-options-1](https://cloud.githubusercontent.com/assets/928116/2600416/df2cd0e4-bafd-11e3-84bc-b2edc581c0b6.png)
![post-options-2](https://cloud.githubusercontent.com/assets/928116/2600418/e3421c3e-bafd-11e3-8b52-f4fd48b0b5d8.png)

## After - view ##
![post-view](https://cloud.githubusercontent.com/assets/928116/2600420/eb8fbef0-bafd-11e3-8ea7-b1bd0dd885b3.png)