Rename ParallelTest::CLI to ParallelTest::Cli.
Make all module methods in ParallelTest::Cli instance methods.

Changes:
- Make ParallelTest::Cli class instead of module.
- Make all module methods of ParallelTest::Cli instance methods.
- Replace across the gem usages of ParallelTest::Cli to
ParallelTest::Cli.new. In fact, there was only one such usage in
bin/parallel_test.
- Fix specs to align them with above changes: Replace usages of
ParallelTest::Cli to subject.
Parametrize ParallelTest::Cli creation with runner.

Changes:
- Add runner parameter to ParallelTest::Cli#initialize method.
- Set runner parameter on calls in bin/parallel_cucumber, bin/parallel_rspec and
bin/parallel_test with appropriate runner class instance.
- Refactor ParallelTest::Cli methods to use instance property @runner
and reduce number of their parameters.
- Refactor --type option handler in OptionParser to find appropriate
runner class and set it or notify that invalid runner name is used.
- Gather runners' require statements in lib/parallel_tests.rb file as
most of them were spreaded through many files and required several times
per run. Now same set of require statements used for all type of
bin/parallel_* files, as well, as in specs.
- Internal calls of the tool to itself just to set appropriate runner
(happened in bin/parallel_cucumber and bin/parallel_rspec) were removed
which have to improve speed of launching of those files.
- Appropriate specs were fixes to satisfy updated code.
Rename ParallelTest::Cli to ParallelTests::Cli.

Changes:
- I do know why, but slightly different root module name was used for
Cli class which was fixed.
Add YARD documentation to constants in runners.
Revert "Rename ParallelTest::CLI to ParallelTest::Cli.".
Merge pull request #184 from zekefast/refactoring_to_improve_cli_class_and_load_speed_of_cucumber_and_rspec_bins

Refactoring to improve cli class and load speed of cucumber and rspec bins