Add a linter rule for parent references

Summary:
Add a linter rule to detect static method calls which should reference the parent class instead of a hardcoded class reference. For example, consider the following:

class SomeClass extends AnotherClass {
  public function someMethod() {
    AnotherClass::someOtherMethod();
  }
}

This should instead be written as:

class SomeClass extends AnotherClass {
  public function someMethod() {
    parent::someOtherMethod();
  }
}

Test Plan: Added unit tests.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D14443