whitespace
[close #56] Raise error on reserved word

'finish' is used internally by Wicked, this commit will raise an error when it is being used externally which would cause complications. 

Will potentially namespace reserved words with `wicked_` in the future.
Change hardcoded strings to constants

Constants allow users to configure first step, last step, and finish step.

The internal `finish` step has been changed to `wicked_finish` to free up that step name for userspace, however this may be a breaking change if you were previously using the string "finish" in your app. You should now use the constant Wicked::FINISH_STEP instead, or you can manually change `Wicked::FINISH_STEP = 'finish'`.

These constants are expected to return strings.