Make the whole project more developer friendly.

Following big improvements were made to make the development more easier:

* Created the 'setup.py' script to support native Python packaging with setuptools and pip.
* There is a new target 'make develop', that will bootstrap the Python virtual environment, install all production and development requirements and install the whole project in editable mode.
* Revised the dependency installation targets.
* Big facelift of makefile help target.
* Added target for build environment cleanup.
* Added target for initialization of new translations of Mentat and Hawat.
* Added target for building locally Python packages.
* Added target for building locally Debian packages.
* Updated documentation page for developers.

(Redmine issue: #4216,#3387,#3361)