REFACTORING Moved core modules to a separate dir.

First step to platform differentiation - we have to split some Windows-specific things in nearest future.
REFACTORING Drop old core.py.

Just forgot to remove it in previous commit.
BUGFIX Do not use ' ... as ...' with zip archieve

Switched to old syntax (open it and close it manually) during compression. We need this to prevent falling on systems with Python v. 2.6 where ZipFile.__exit__ attribute was not implemented yet. Unfortunately one of the target platforms has this old Python.
DOC Changed Python requirements.