Flags and Help

• You can now print usage with retest --help
• You can now use flags to tell retest which standard ruby test command to use
• Update README documentation

Shortcut Flags

flags	shortcut command
retest --rake	bundle exec rake test TEST=<test>
retest --rake --all	bundle exec rake test
retest --rspec	bundle exec rspec <test>
retest --rspec --all	bundle exec rspec
retest --rails	bundle exec rails test <test>
retest --rails --all	bundle exec rails test
retest --ruby	bundle exec ruby <test>