implementing changes based on feedback on PR #1527

changed "examplegroup" variable to be "example_group"

modified wait_for_completion methods in example_thread_runner.rb and example_group_thread_runner.rb to join to the sub-threads instead of polling

modified wait_for_available_thead method in example_thread_runner.rb to only sleep for 0.1 seconds instead of 1 second to reduce wait overhead while polling for an available thread to run within

changed doublequotes to singlequotes in "spec/command_line/parallel_spec.rb" to avoid the need for escaping