Expanded - Refactor authorization - Add protected fetch for private gems

[rjocoleman]

This builds on #92, #93 and resolves #24

@smellsblue I think this covers off all your feedback - thanks!

This PR contains:

• Iteration on @smellsblue's work on the Authorization refactor
• protected_fetch (default false) in Gemstash::Configuration
• Configuration of protected_fetch via $ gemstash setup
• A new fetch permission for Authorization
• Authentication via HTTP Basic Auth (token as username) to support gem, bundler and uri based auth in Gemfiles
• Docs to suit.

[rjocoleman]

@smellsblue if this is acceptable I'll do the serve_gem and serve_marshal methods in a similar way and stick them in this PR too.

[rjocoleman]

@smellsblue Pending any further feedback, this should be good to go