If you are going to use a battery powered contact sensor be careful if you are in an area where it gets cold in the winter. Garage doors are notoriously drafty. Having a battery operated device basically directly in the path of sub-zero wind blowing in will have a big impact on performance of the device. I had to use an AC powered solution in the end. All the contact sensors I tried just wouldn't work. So, I use the Hubduino project to wire a sensor in. Also, you can use a relay with that board to control your garage door. There is a prefab driver that integrates the two devices at the board level. If you're a DIY and have any experience with Arduino it might be something to consider.