Add config.ru

Now that we use the `Sinatra::Base` application, we have to tell the
server to run our application class. We do this by specifying a Rack
config file. Now instead of running

    $ ruby app.rb

to start the app server, we run

    $ rackup

1 files changed, 3 insertions, 0 deletions

diff --git a/config.ru b/config.ru
new file mode 100644
index 0000000..eae1e13
--- /dev/null
+++ b/config.ru
@@ -0,0 +1,3 @@
+require './app'
+
+run App