introducing FactoryBro, AddonFactory, and AppFactory

let's you create `Addon` objects (unsaved or saved) with some nice defaults. this is an improvement to the previous `addon_factory` which did 4 saves. and it speeds up tests a tad:

     (z)(Chris-MacBook-Pro.local) %% python /opt/speed.py factories "webapps search"                                                                                                    /opt/zamboni (factories)Benchmarks for webapps search:
     After (factories):
          Dropping database ...
          Trial 1:
               Ran 181 tests in 33.986s
          Dropping database ...
          Trial 2:
               Ran 181 tests in 31.541s
          Dropping database ...
          Trial 3:
               Ran 181 tests in 31.702s
          Dropping database ...
          Trial 4:
               Ran 181 tests in 31.457s
          Dropping database ...
          Trial 5:
               Ran 181 tests in 31.730s
          Total duration: 160.4160s
          Average time per trial: 32.0832s
     Before (master):
          Dropping database ...
          Trial 1:
               Ran 181 tests in 41.873s
          Dropping database ...
          Trial 2:
               Ran 181 tests in 43.294s
          Dropping database ...
          Trial 3:
               Ran 181 tests in 39.206s
          Dropping database ...
          Trial 4:
               Ran 181 tests in 35.471s
          Dropping database ...
          Trial 5:
               Ran 181 tests in 35.499s
          Total duration: 195.3430s
          Average time per trial: 39.0686s
     Before (195.3430 / 5) vs. After (160.4160 / 5) => 6.9854s faster (1.2177x speedup)