Refactor experiments module

I refactored `skll.experiments` to fix #73 by limiting the number of processes we're spawning at any given point in time. I also simplified a number of things along the way.

The changes are:
-  Ablation involves loading a single configuration file and then generating modified featuresets, instead of 
   making many temporary configuration files.
-  Deprecated `run_ablation`, because it's been folded into a few lines in `run_configuration` 
-  Can now have multiple featuresets in a config file that is being used for ablation, which fixes #49.
-  Removed ability to run multiple configuration files in parallel with `run_experiment` when in `gridmap` mode.
   Now each is run sequentially.