Simple Safety Geofence

This implements a simple geofence module that provides a safety radius. When you exceed the warning
radius an alarm is set. When you exceed the error radius your motors will shut down. This module prioritizes
not going outside the boundary of the geofence over the model safety.

To address a few points that came up on IRC. I consider this the safety check of last resort
- fly in a field that is unpopulated
- set the radius smaller than that field
- you won't hit people

This is to encourage safe flying behavior. It also covers any bugs in the code (or piloting) that might result in a flyaway.

In the future when navigation is 100% robust we can enable RTH or landing when hitting the warning
boundary but I would like to keep the scope of this to the simple goal of not flying away ever. In addition this means if we do something like enable RTH as a failsafe mode but something goes wrong, it won't ever fly away.

This also adds a simple setting of the radius to the Modules page:
![screen shot 2013-12-19 at 12 52 22 pm](https://f.cloud.github.com/assets/618346/1785186/d83414a2-68df-11e3-8863-a6c5e2dc815d.png)

As a note, #604 and #648 could be merged as well as an alternative GeoFence module (either compile
option or a separately named OPT_MODULE). Right now they are not viable alternatives as there is not
an easy way for end users to provide a simple boundary but rather it is for more sophisticated boundaries
that are created in an external application. They can easily exist in parallel simply by a consistent API - communicate between from the Goefence module to the control logic through the SystemAlarms.Alarm[Geofence].

Here is an example of it working:
<iframe src="//player.vimeo.com/video/82419090" width="500" height="281" frameborder="0" webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe> <p><a href="http://vimeo.com/82419090">TauLabs Geofence testing</a> from <a href="http://vimeo.com/user4648573">James Cotton</a> on <a href="https://vimeo.com">Vimeo</a>.</p>