Allowing manual marking of devices that are for demo/testing purposes only

This mostly addresses issue #671.

Background:
Currently, as we look more closely at our data, it is important to separate "real" and "demo" data.  Here's some fake data, on our real servers, accounting for ***over 20% of our exercise data and about 33% of our video data.***
![image](https://f.cloud.github.com/assets/4072455/1103432/7ecec060-18a8-11e3-8a43-5314777d535c.png)

We could try to clean data, but some of our demos / advocates require syncing to the central server, and we wouldn't want to destroy their data.  In addition, outside of here, nobody has the possibility of marking their data as "not real"--they either have to ignore, or to create a new zone.

Changes:
An easy way to address is to add a field to DeviceMetadata, "is_demo_device".  We can expose a simple checkbox for setting this on the central server, if we like--here, it must be set manually as admins.

Utility:
This field can be then used in database queries, to easily filter out demo data from the results that we report.

Testing:
Tested manually.  Small change, works like a champ!  I've also marked this in the control_panel app, such that people can see what data has been marked as "demo" data

![image](https://f.cloud.github.com/assets/4072455/1103439/ccce40c4-18a8-11e3-9172-3b6497e6ef9c.png)