Allow specifying a list of checks that should run.

This reverses the default assumption of "everything enabled"
fix copy paste error
spec for --only-check
Add a load_spec_helper convenience method for testing external plugins
Test against ruby 2.0.0 and 2.1.0
Force rubygems 2.1.11 on travis
Don't try and install ruby-prof on 1.8.7
Replace the janky have_problem matchers

This removes the kind of janky have_problem and only_have_problem
matchers with the new contain_(warning,error,fixed) matcher. This
greatly improves the readability of both the tests and their output on
failure.
Convert the plugin specs to the new matcher
Convert the rest of the specs to the new expect format
Update nested_classes_or_defines message

This annoyed me when I was fixing the specs, so I’m going to change it
now.
Update this spec too
Merge pull request #238 from rodjek/rejig_tests

Rejig tests
Merge pull request #228 from rodjek/only_run_certain_checks

Allow specifying a list of checks that should run
Remove check explanations from README.md
Test the values passed to PuppetLint::CheckPlugin#notify
Merge branch 'master' into move_output_out_of_lib

Conflicts:
	lib/puppet-lint/checkplugin.rb
	spec/puppet-lint/plugins/check_classes/autoloader_layout_spec.rb
	spec/spec_helper.rb