Refactor `troo show <resource> <id>`;

- Reduce duplication in this class.
- Remove Troo namespace where safe to do so.
- Remove pointless (empty/nil) returns from stubs.
Further refactoring;

- Remove instance variables which absent-mindedly crept in.
- Reduce duplication further by extracting resource retrieval out to module.
- Move ThorFixes class into CLI::Helpers for better organisation.
Further refactoring;

- Call #presenter on 'resource' to render itself.
- Call #decorator on 'resource' to decorate itself.
- Move setting of default into Helpers as it is used in multiple CLI commands.
- Add options to CommentDecorator and MemberDecorator to provide symmetry.
- Rename #render_show to #show.
- Minor copy and whitespace changes.
Merge pull request #8 from gavinlaking/feature/duplication

Refactor `troo show <resource> <id>`;