Add linode.clone

Just needed the 'linode.clone' method and found it was not there. This is what I did to add support for it.

BTW: I could not run the specs. Among many 'mocha' deprecation warnings I finnally got an error. Either in ruby1.9.3 or in ruby2.0.0:

```
*** Mocha deprecation warning: Change `require 'mocha'` to `require 'mocha/setup'`.


*** Mocha deprecation warning: Test::Unit or MiniTest must be loaded *before* Mocha.


*** Mocha deprecation warning: If you're integrating with a test library other than Test::Unit or MiniTest, you should use `require 'mocha/api'` instead of `require 'mocha'`.


*** Mocha deprecation warning: `require 'mocha/standalone'` has been deprecated. Please use `require 'mocha/api' instead.


/home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/mocking/with_mocha.rb:2:in `require': cannot load such file -- mocha/object (LoadError)
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/mocking/with_mocha.rb:2:in `<top (required)>'
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/configuration.rb:164:in `require'
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/configuration.rb:164:in `mock_framework='
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/configuration.rb:123:in `mock_with'
	from /home/ruben/Documentos/codigo/linode/spec/spec_helper.rb:21:in `block in <top (required)>'
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core.rb:56:in `configure'
	from /home/ruben/Documentos/codigo/linode/spec/spec_helper.rb:20:in `<top (required)>'
	from /home/ruben/Documentos/codigo/linode/spec/linode_spec.rb:1:in `require'
	from /home/ruben/Documentos/codigo/linode/spec/linode_spec.rb:1:in `<top (required)>'
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/configuration.rb:387:in `load'
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/configuration.rb:387:in `block in load_spec_files'
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/configuration.rb:387:in `map'
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/configuration.rb:387:in `load_spec_files'
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/command_line.rb:18:in `run'
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/runner.rb:55:in `run_in_process'
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/runner.rb:46:in `run'
	from /home/ruben/.rvm/gems/ruby-1.9.3-p429@linode/gems/rspec-core-2.4.0/lib/rspec/core/runner.rb:10:in `block in autorun'
rake aborted!


```