Global replace Sets by Lists (#559).

BREAKING CHANGES

Core API:

- Rename class ResultSet and change package
  `de.deepamehta.core.ResultSet`
  -->
  `de.deepamehta.core.service.ResultList`

- Drop ResultList's method `getIterator()`.
  Use `iterator()` instead.

Global:

- Core Service API and DeepaMehtaObject API thoroughly uses Lists instead of Sets.
  A lot of methods are concerned. Too much to be listed here.
  -> You must adapt your plugins. Adaption should be straight forward.

  One reason for this change:

  > HashSet consumes about 5.5 times more memory than ArrayList for the same number of elements (although they're both still linear), and has significantly slower iteration (albeit with the same asymptotics); a quick Google search suggests a 2-3x slowdown for HashSet iteration versus ArrayList.
    http://stackoverflow.com/questions/10799417/performance-and-memory-allocation-comparision-between-list-and-set

See #559.