Added DocTest tab to main jsduck page.

Rakefile

lib/jsduck/app_data.rb

lib/jsduck/options.rb

template/app/Application.js

template/app/Assert.js

template/app/History.js

template/app/controller/Content.js

template/app/controller/DocTests.js

template/app/controller/InlineExamples.js

template/app/controller/Tabs.js

template/app/model/DocTest.js

template/app/view/Viewport.js

template/app/view/doctests/Index.js

template/app/view/examples/Inline.js

template/app/view/examples/InlinePreview.js

template/eg-iframe.html

template/resources/sass/_doc_tests.scss

template/resources/sass/viewport.scss