Switch to lightgbm

This is a major release, with breaking API changes:

• The random forest package is now lightgbm
  • Much more lightweight (serialized kernels tend to be 5x smaller or more)
  • Much faster on big datasets (for comparable parameters)
  • More flexible... We can now use gbdt if we wish. lightgbm is more flexible in general.
• Added a mean_match_subset parameter. This will help greatly speed up many processes.
• mean_match_candidates now lazily accepts dicts as long as the keys are a subset of parameters in variable_schema.
• Model parameters can be specified by variable, or globally.
• Mean matching function can be overwritten if the user wishes.