Reduce global scope pollution.

A few custom exception classes are defined at the global scope. There's
no need! Nest 'em inside the `RedButton` class, and they're still
addressable within it without further modification. Note, however, that
the RSpec examples must refer to, e.g.,
`RedButton::MalformedTargetsError` rather than the naked
`MalformedTargetsError`.

I also move the `include "curses"` call into RedButton itself, since
only `RedButton` needs access to the methods therein.

Lastly, I renamed `the-red-button.rb` to `red_button.rb` because it's
more common Ruby usage.