Refactor to App instance, rather than Ryespy singletons.

Introduce `App`, and configure on an instance rather than `Ryespy`
itself, moving related code from `lib/ryespy.rb` to `lib/ryespy/app.rb`.
Pass config, redis connection, notifiers, and logger where necessary.

Also move some `Config` tests into new `App` tests.
Simplify internal Ryespy::X namespace calls to X, etc.
Tweak executable --help, --version display.
Merge branch 'refactor'