The logs are always stored in log/bullet.log
If you run the specs with `BULLET=true bin/rspec`:
* Any feature test which makes bullet angry will fail
If you run rails with `BULLET=true bin/rails s`:
* It will print the bullet logs in both the rails log and the bullet log
* It will show a footer on each page with the N+1 queries etc.
This commit allows us to use caching and Rails.cache.
Temporarily we configure the null store in all environments,
but those will probably be memcached in the future.
References #235
