Skip the creeper auto-play test

For now, we're generally avoiding implementing creepers until after the school
project in the interest of time, so skip the test.
Convert a player's hand to a Set

Use ruby's standard library instead of re-inventing our own around a normal
Array
Further meld the Ruleset class

- Rename draw_limit to draw_count
- Add a goal accessor
- Create InvalidRuleError

Which converts a bunch of tests from Error to Fail, which is a step in the right
direction, or so.
Remove a bunch of trailing whitespace

Unnecessary! Unneeded! Easy to remove, even in vim:

   :%s/\v\s+$//g

And, done!
Raise NotImplementedError in the default play

Force each subclass to define their own #play(ruleset, player) method.
Don't call super

Especially now that it will raise an error :D
Add some validation to rule cards
Validate a Rule card

A rule card must modify a rule as well as the new value for the rule
Forgot to call 'super'

The values for name, description, etc aren't set until the parent class
initialize is called as well, which means that the name was always 'nil'