Initial plumbing of nested diagnostic context
Changed Message to use an args parameter to make future changes non-breaking
Added support for the old Message constructor for backward compatibility
Move NestedDiagnosticContext into its own file and documented everything
Utilized NDC in the default formatters
Incremented version to 0.2.0
Added Rack middleware

Middleware ensures that the nested diagnostic context is cleared between
requests. It is added by default by the Railtie.
Updated RELEASE.md for 0.2.0