Now the name of an argument that causes assertion failure should be
obtainable.

This includes mode dictionary creation, removal of 'mode' member
from args_to_add dictionary and repositioning of the latter for
the sake of consistency.

This way the code should be more flexible and easier to read.

Both short and long versions of defining an argument are doing
the same thing and are processed by argparse. Therefore, it is
presumed that testing both versions of defining each argument
is enough and overriding tests can be done just for one version.

Note: new version includes mode args check that was missing.

In current version the line of code is pointless because arg
is either 'source' or 'dest' which are redefined in the next few
lines. However, the code reader may not know that. Furthermore,
this line may be of use if it would be necessary to test
some other arguments' interaction with hdfs.

tearDown() and check_args() were modified to accomodate
the changes.

The argument differs from the others because its type is not
string - therefore, argparse performs some manipulations with
the supplied file, and these manipulations have to be taken
into consideration.

Argparse module throws SystemExit when something is wrong with the
supplied arguments, regardless of the problem's type. Implemented a
function to intercept such behaviour and retrieve argparse' message.
Analysis of the message is used to check that the encountered
problem is the expected one.

These arguments have a fixed sets of allowed values. The tests
are checking that the code correctly displays error information
when an unexpected argument value is supplied.

The functions now perform 'short'-related manipulations by
defining varialbes and using them in functions instead of
having two different calls of the same function.

Now the tests check that the parameter in question is a file,
rather than not None.

- Replace stage.parse_args() with stage.configure().
- Update tests to set input_dir if source is either stream or hdfs.
- Add an input file name into most tests - 'file' is now a default source, and
  it demands an input file. Exceptions: test_i/test_input_dir - all files in
  the input dir will be taken, so input file is not mandatory; all tests
  including --hdfs; test_default - its role is to simulate an execution with
  no command line arguments.
- Change test_default() to catch the error about missing input file.
- Change add_override_mode() - file source/destination is not allowed in
  map-reduce mode, meaning that such combination raises an error that must be
  caught.
- Define stage.configure() arguments in list args in some tests to follow
  the 80 symbols line length restriction without stretching a single code line
  into 3+ lines.

If EOM is set to empty string, EOP will be set to '\n', unless overridden.

It is possible that value 'i' will become valid in the future, while
'incorrect' is much 'safer' in this regard - all of the values are
single letters.

- Move common function to separate file lib.py.