Add hooks for Trial#choose! and Trial#complete!

This adds two new attributes to Split.configuration,
on_trial_choose and on_trial_complete.

Set these attributes to a method name available in the
same context as the ab_test method. These methods should
accept one argument, a Trial instance.

For example:

    Split.configuration.on_trial_choose = :log_choice

    # application_controller.rb
    def log_choice(trial)
      logger.info "experiment=%s alternative=%s user=%s" %
        [ trial.experiment.name, trial.alternative, current_user.id ]
    end
Update readme with trial event hook documentation
Merge pull request #176 from bmarini/trial-hooks

Add hooks for Trial#choose! and Trial#complete!