Added average time scoring in both single game and main scorer

fixes #15 Here is the migration script. Figured that we don't need the avg_time in the last run, just like the score, it only exists inside `best_score` object
```
import pymongo

db = pymongo.MongoClient().battleships # connect to local instance

SOME_MEANINGFUL_DEFAULT = 0

for bot in db.bots.find():
    bot["avg_time"] = SOME_MEANINGFUL_DEFAULT
    db.bots.save(bot)

for user in db.users.find():
    # not all users have a best and last score
    if "best_score" in user:
        user["best_score"]["avg_time"] = SOME_MEANINGFUL_DEFAULT
    db.users.save(user)
```