Add Rack-based convenience methods to Rack::Console

This adds a method `app` that will return the underlying Rack
application wrapped in a test session. Thus you can perform fake requets
to your application:

```ruby
pry(main)> app.get('/')
=> #<Rack::MockResponse:0x007f97bcd5b2a8 @status=200, @body=["Hello world!"]>
```

Additionally, fix a few issues:
 * Move `reload!` (and `app`) into a Rack::Console::Methods module that
   only gets included into the `TOPLEVEL_BINDING` (main)
 * Clean up the test suite!

Signed-off-by: David Celis <me@davidcel.is>