Description of problem:

When getting the list of all test plan cases it would be fine to
include the sortkey as well. I see there is the TestCasePlan.get()
function but it would more efficient to fetch all the sortkeys in
one call rather then calling it for each test case separately.

Version-Release number of selected component (if applicable):
Nitrate 3.8.0

Steps to Reproduce:
TestPlan.get_test_cases(6544))

Actual results:
[{'alias': '',
  'arguments': '',
  'attachment': [],
  'author': 'psplicha',
  'author_id': 2117,
  'case_id': 174452,
  'case_status': 'CONFIRMED',
  'case_status_id': 2,
  'category': '      Function    ',
  'category_id': 415,
  'component': [],
  'create_date': '2012-07-07 03:19:43',
  'default_tester': 'psplicha',
  'default_tester_id': 2117,
  'estimated_time': '00:00:00',
  'extra_link': None,
  'is_automated': 0,
  'is_automated_proposed': False,
  'notes': '',
  'plan': [6740, 6730, 6729, 6726, 6725, 6544, 6504, 6503, 6499],
  'priority': 'P1',
  'priority_id': 1,
  'requirement': '',
  'reviewer': None,
  'reviewer_id': None,
  'script': '',
  'summary': 'Test case',
  'tag': []},
  ...

Expected results:
sortkey included in the result hash as well.