Version 0.20.0

• Refactored experiments module to remove unnecessary child processes,
  and greatly simplify ablation code. This should fix issues #73 and #49.
• Deprecated run_ablation function, as its functionality has been folded
  into run_configuration.
• Removed ability to run multiple configuration files in parallel, since
  this lead to too many processes being created most of the time.
• Added ability to run multiple ablation experiments from the same
  configuration file by adding support for multiple featuresets.
• Added min_feature_count value to results files, which fixes #62.
• Added more informative error messages when we run out of memory while
  converting things to dense. They now say why something was converted to
  dense in the first place.
• Added option to skll_convert for creating ARFF files that can be used
  for regression in Weka. Previously, files would always contain non-numeric
  labels, which would not work with Weka.
• Added ability to name relation in output ARFF files with skll_convert.
• Added class_map setting for collapsing multiple classes into one
  (or just renaming them). See the
  run_experiment documentation for details.
• Added warning when using SVC with probability flag set (#2).
• Made logging much less verbose by default and switched to using
  QueueHandler and QueueListener instances when dealing with
  multiple processes/threads to prevent deadlocks (#75).
• Added simple no-crash unit test for all learners. We check results with
  some, but not all. (#63)