first stats view
added stats.rb controller

Assumed that session[:round_id] is available.

Retrieved the relevant round from the DB using the round id.

Calculated the following from the round:
- deck id
- number of correct answers
- number of incorrect answers
- number of skipped answers
- percentage of correct answers

Passed this data to the stats view (stats.erb) as instance variables.
a newer version already exists on another branch.
Merge pull request #3 from CharlieTruong/stats_controller

Stats controller