Add Guard testing support

While developing rack-attack, run tests continuously with `bundle exec
guard`

Gemfile

@@ -1,4 +1,9 @@
-source "https://rubygems.org"
+source 'https://rubygems.org'
+
 gemspec
 
-#gem 'debugger', platform: 'ruby'
+group :development do
+  gem 'pry'
+  gem 'guard' # NB: this is necessary in newer versions
+  gem 'guard-minitest'
+end

Guardfile

@@ -0,0 +1,10 @@
+# A sample Guardfile
+# More info at https://github.com/guard/guard#readme
+
+guard :minitest do
+  # with Minitest::Spec
+  watch(%r{^spec/(.*)_spec\.rb$})
+  watch(%r{^lib/(.+)\.rb$})         { |m| "spec/#{m[1]}_spec.rb" }
+  watch(%r{^spec/spec_helper\.rb$}) { 'spec' }
+end
+