Refactor data access and Ruleset

[dinoboff]

• Separate RuleDataSnapshot methods by purpose:

  • Snapshot methods have been left in the RuleDataSnapshot class;
  • Methods to convert data from/to a firebase data three are in a new DataNode class;
  • Methods to manipulate the data in the Database class.

• Refactor Ruleset:

  • Shared method to evaluate read and write permission (via RuleNode#$traverse).
  • Shared method to set wildchildren (via RuleNode#$child(path)).
  • Shared methods report the result (via Result).
  • Separate logic to parse the ruleset (RuleNode and Rule), the logic to walk the rules and evaluate them to simulate operation (RuleSet), and the logic to report the results (Result).

Fix #73