allow Creators to throw fillingException, added new Exception

* allow the different Creators to throw FillingExceptions 
   by declaring the Exception in the interface
* added NoEnumConstantsException for Enum classes that 
  do not have any Enum constants defined
* SimpleEnumCreator can now throw NoEnumConstant exception
updated EnumTest

* ensure that the NoEnumConstantsException is thrown
  if an Enum does not have Enum constants defined
added new method to RandomGenerator

* added getRandomIntBetween(min, max) for convenience
added tests for the RandomGenerator

* added tests for getRandomIntBetween(min, max) as it 
  is not only a wrapper for a java.util.Random method,
  but has an actual implementation
* added PowerMock as test dependency to be able to mock
  static methods
updated StringCreator

* it now uses the newly created 
  RandomGenerator.getRandomIntBetween(min,max) method
added tests for StringCreator

* ensure that the min and max length are used correctly
* ensure that the charset is used completely