implement getStringOfCombinationIndex()
implement getStringOfCombinationIndex()

by given vector<string> elements,
return a string of possibilities.

ex)
  vector<string> elements;
  elements.push_back("a");
  elements.push_back("b");
  elements.push_back("c");
  elements.push_back("d");
  elements.push_back("e");

  // elements.size() == 5, meaning that it's quinary(base 5) notation.
  getStringOfCombinations(elements, 0) == a
  getStringOfCombinations(elements, 4) == e
  getStringOfCombinations(elements, 5) == aa
  getStringOfCombinations(elements, 54) == aee

ivere27/SimpleCertificateManager