Extract device-specific code into a Device module, rename NestThermostat namespace to just Nest

[stevenpetryk]

The new hierarchy should be:

module Nest
  # Handles authorization and API logic
  class Nest; end

  # Stores structures
  class Structure; end

  # Device mixin
  module Device; end

  class Thermostat; include Device; end
end

Using Device as a module seems more appropriate, since Device is technically an abstract class.

Calls could look something like this:

nest = Nest::Nest.new(email: 'email', password: 'password')
nest.structures # => [Structure, Structure]
nest.structures.first.thermostats # => [Thermostat, Thermostat, Thermostat, ...]

# Not certain whether there should be a way to shallowly access the thermostats too
nest.thermostats # => ALL thermostats

[stevenpetryk]

Trying to actually write this, I decided it'd be best to make Device its own class.

module Nest
  class Nest; end
  class Structure; end
  class Device; end
  class Thermostat < Device; end
end