adding basic statistics

this is still WIP but we have some basic statistics endpoints and the beginning of an UI. 

# statistics endpoints
api.add_resource(ApiChallengeStats,
                 '/api/stats/challenge/<string:slug>')
api.add_resource(ApiChallengeStatsPerUser,
                 '/api/stats/challenge/<string:slug>/users')
api.add_resource(ApiStatsUserTotals,
                 '/api/stats/users')
api.add_resource(ApiStatsChallengeTotals,
                 '/api/stats/challenges')
api.add_resource(ApiMe,
                 '/api/stats/me')