Devtools

Added [devtools](https://github.com/rom-rb/devtools).

```
rake build                      # Build active_model-better_errors-1.6.4.gem into the pkg directory
rake ci                         # Run all specs, metrics and mutant
rake ci:metrics                 # Run metrics (except mutant)
rake install                    # Build and install active_model-better_errors-1.6.4.gem into system gems
rake metrics:coverage           # Measure code coverage
rake metrics:flay               # Measure code duplication
rake metrics:mutant             # Measure mutation coverage
rake metrics:reek               # Check for code smells
rake metrics:rubocop            # Check with code style guide
rake metrics:yardstick:measure  # Measure docs in  with yardstick
rake metrics:yardstick:verify   # Verify that yardstick coverage is at least 100%
rake release                    # Create tag v1.6.4 and build and push active_model-better_errors-1.6.4.gem to Rubygems
rake spec                       # Run all specs
rake spec:integration           # Run integration specs
rake spec:unit                  # Run unit specs
rake yard                       # Generate YARD Documentation
```

Now we get a bunch of metrics to measure code quality, check code smell, etc.