Robot name

Robot name exercise implemented. Default name generator now persists until name collision is resolved.
Better Idea.

```ruby
def error_message
  @error_message ||= "Tried to re-generate previously generated Robot Name"
end
```

An even better idea is to stick this in the initializer:

```ruby
def initialize(msg = nil)
  @message ||= "Tried to re-generate previously generated Robot Name"
end
```

... thus eliminating a number of redundant unnecessary methods.