Move all development dependencies to the Gemfile

And only require they be installed on CRuby, since
they do not work too well on MacRuby.
Add a lazy loading flog task and yard task
Fix some documentation that YARD warned about
Update demo code
Add the gemnasium badge to the README
Fix another YARD warning
Add YARD task

The lazy way, because the task is a pain to
configure and forking is "cheap" on CRuby.
Update the Accessibility.version test
Refactor test suite and how tests are run

Remove the test:string task as it is pointless now.
The code for Accessibility::String will move to its
own gem soon. The test file can be run standalone
in the mean time.
Merge remote-tracking branch 'ax/master'

* ax/master:
  Remove a use of return in element.rb
  Fix next_version.jpg link