Initial sinatra project structure

Rakefile

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+require 'bundler/setup'
+
+namespace :server do
+  task :start do
+    ENV['APP_ENV'] = 'production'
+    system("puma")
+  end
+
+  task :dev do
+    system('rerun --no-notify --exit --dir="src/" puma')
+  end
+end
+
+namespace :test do
+  task :rubocop do
+    system("rubocop src/")
+  end
+end