Added auto-retry function

Adds a new command that will automatically retry any failed specs after the initial test run, without having to explicitly call ```respec f```.  Instead, you can call ```respec r``` and any specs that fail during the initial run will be rerun exactly once.

Also updates ```README``` and tests to reflect these changes, as well as changes a few small things like removing ```should``` from ```it``` block descriptions, to be more in line with most recent rspec standard best practices.

**One important caveat:**
Changing ```exec``` to ```system``` in ```bin/respec``` seems to be messing with the return status in ```spec/integration_spec.rb``` so that ```output``` is the same, but ```status``` is always true.