Rudimentary rake-based benchmarking

Gives us a simple base from which to make perf-related changes. At time of
commit, default 1000-run benchmark results were:

vagrant@development:/var/govuk/bouncer$ bundle exec rake benchmark:all
              user     system      total        real
no host:  1.020000   0.080000   1.100000 (  1.434709)
410:      0.690000   0.050000   0.740000 (  1.013773)
301:      0.700000   0.040000   0.740000 (  1.024336)
404:      0.750000   0.000000   0.750000 (  1.045783)
Use ActiveRecord::QueryCache as middleware

Avoids repeated queries to the database for queries whose strings are identical
in the context of the current request.

vagrant@development:/var/govuk/bouncer$ bundle exec rake benchmark:all
                user     system      total        real
  no host:  0.940000   0.070000   1.010000 (  1.152042)
  410:      0.660000   0.010000   0.670000 (  0.777661)
  301:      0.660000   0.010000   0.670000 (  0.767946)
  404:      0.650000   0.010000   0.660000 (  0.768585)
Merge pull request #99 from alphagov/performance-tweaks

Don't repeat ActiveRecord queries