Make calls between robots

Make it easy to make calls between robots.

Assuming there are 2 different robots named "robot 1" and "robot 2", the one named "robot 1" can call any of the other robot's methods by referring to it by name using the `Master.robot` method like this:

```ruby
robo2 = Artoo::Master.robot("robot 2")
robo2.hello("hi there")
```