Autoload mapping strategies

Loading faraday is slow and only required for certain strategies.

To speed load time we now autoload strategies.

Closes #26 (closed)

Impact on gitlab-org

Before

$ time be tff -f tests.yml app/models/project.rb
spec/models/project_spec.rb

real    0m1.154s
user    0m0.626s
sys     0m0.528s

After

$ time be tff -f tests.yml app/models/project.rb
spec/models/project_spec.rb

real    0m0.530s
user    0m0.457s
sys     0m0.086s