Record results of RuleEngine executions (django)

[miraculixx]

This story will keep track of all rules executed and store results.

1. Implement a pyrules.Rule.store method to save the ._executed trace
2. Add models.RuleResults to keep results of executions. Attributes rule name, input context, output context, datetime started, ended, rule._executed dump
3. Make storing rule executions a settings.py flag. If the flag ist true, Rule.store should save its result and trace to models.RuleResults.