Add a new variant of detectMultiScale with output arguments 'objects' and 'numDetections'; factor most of detectMultiScale's code into 2 protected methods

Python 3 requires METH_VARARGS | METH_KEYWORDS, not just METH_KEYWORDS.

In these cases, just using print() is enough to ensure py2/3
compatibility.

Added a header with defines to enable the module to compile with either
python 2 or 3 without changes.

* converted it to Algorithm
* old API still can be used for source compatibility (marked as deprecated)

* converted it to Algorithm, because implementation uses inner buffers and
  requires preprocessing step
* new implementation splits preprocessing and transform,
  what is more effecient
* old API still can be used for source compatibility (marked as deprecated)