Move gemspec into puppetlabs_spec_helper.gemspec

This was originally removed in 96108738bba5e69e385858c1b4edfc1c900b9aab because
it wasn't been managed. For some reason there was a rake file to generate the
gemspec but this isn't useful when you want to include this gem in a project.
Remove gem dependencies in Gemfile

There is no need to source the gems in the Gemfile if they're already declared
in the gemspec.
Remove executable bit from files that do not require it
Consistently use PuppetlabsSpecHelper
Add test strategy to Rakefile
Move lib/puppetlabs_spec_helper for gem require
Get tests passing
Fix rake_tasks
Ignore vendor
Add rubygems release rake tasks

puppetlabs_spec_helper(feature/add_gemspec*)$ bundle exec rake -vT
rake build    # Build puppetlabs_spec_helper-0.4.1.gem into the pkg directory
rake install  # Build and install puppetlabs_spec_helper-0.4.1.gem into system gems
rake release  # Create tag v0.4.1 and build and push puppetlabs_spec_helper-0.4.1.gem to Rubygems
rake spec     # Run RSpec code examples
Update README