By using FactoryGirls's build strategy :build in associations the time penalty of using FactoryGirl
instead of ActiveRecord stubs is down to a factor < 2.

  - Clean object cache before each example with type :faster
  - Started to work on FactroyGirl integration

N.B. While FactoryGirl support right now only includes defaults it slows down the ActiveRecord based stubbing by a factor of ~2.5
which might be fixable if worth it.

Run with
        N=..., NO_RCOV= bundle exec spring rspec   --example 'stubbed' ./spec/models/faster_specs_spec.rb
or
        N=..., NO_RCOV= bundle exec spring rspec   --example 'in DB' ./spec/models/faster_specs_spec.rb

the benchmark result in seconds were:

     N:     |   10  |   20  |   40  |   80  |
------------+-------+-------+-------+-------+
   'in DB'  |   32  |   59  |  116  |  230  |
------------+-------+-------+-------+-------+
 'stubbed'  |  3.4  |  3.5  |  3.9  |  4.5  |
------------+-------+-------+-------+-------+

Considering warm up times that gives stubbed ~0.01s/N
and in DB ~2.8s/N or a factor of ~280