Added simplecov

I've started to use simplecov on json-schema to find gaps in the test
coverage. This might be useful to other developers too, so I thought it
would be useful to share the simplecov config I'm using

.gitignore

@@ -5,3 +5,4 @@ pkg
 /Gemfile.lock
 .bundle
 .idea
+/coverage

Gemfile

@@ -3,3 +3,4 @@ source "https://rubygems.org"
 gemspec
 
 gem "json", ">= 1.7", :platforms => :mri_19
+gem 'simplecov', :require => false

test/support/test_helper.rb

@@ -1,3 +1,10 @@
+if ENV['COVERAGE']
+  require 'simplecov'
+  SimpleCov.start do
+    add_filter '/test/'
+  end
+end
+
 require 'minitest/autorun'
 require 'webmock/minitest'