Refactoring: English class & variable names, move constants to one class

This refactoring was rather trivial because we have unit tests
preventing regressions. BTW, the structural refactoring is still our
biggest TODO...
Make all CLI flags constants and convert args into a Vector

Why a vector and not just keep the array? I guess in the next iteration
methods like 'contains' and 'indexOf' might come in handy.
Maven cosmetics: explicitly use UTF-8 encoding
Final en bloc refactoring: get rid of convoluted parameter parsing

- Rename constant UNDEFINED to ALL_CUSTOMERS because that is what it
  means
- Parse month parameter in format yyyy-MM using SimpleDateFormat, but
  re-convert it to a String because OrderAggregator needs it in that
  format (the assumption during the refactoring was that the
  functionality behind OrderAggregator is a black box)
- Improve usage message by explaining the parameters and the fact that
  parameter order is not prescribed anymore. Furthermore any unknown
  parameters before, in between or after known parameters are now just
  ignored.
- Unit tests have been updated to reflect the new situation. They still
  can be regarded as the functional specification for the coommand line
  interface.