Add local selenium config file. Fixes #3118.

  Selecting the --local option, or setting PERSONA_NO_SAUCE=1 in the
  environment, will now switch to a different config file. The set of
  supported platforms will be totally generic.
Show how to run >1 firefoxen locally. Fixes #3138
Improve explanation of using multiple Firefoxen
Merge pull request #3142 from 6a68/issue_3118_local_selenium

Add separate selenium config for local runs, including multiple FF channels. Fixes #3118, #3138.

AHHA! I get it now. I totally dig this.

r+
changes in train-2013.03.29
bump train version in dev to train-2013.04.12
Remove stray underscore leftover. Might fix #3153...