(Redmine issue: #3387)

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)

This commit represents huge amount of work and introduces new Python-based version of Hawat web user interface for Mentat system. It is built on top of the great Flask framework. Currently it is still work in progress and many features are implemented only as dummy stubs, however the core functionality is almost finished. Following is a list of current key features:

  * Focus on modular architecture using Flask`s blueprint feature.
  * Multi-layered configurability (defaults, multiple file overrides).
  * Plugin system.
  * Internationalization support.
  * Support for multiple authentication systems.

This commit provides working albeit hollow starting point for future web interface development. IT IS NOT READY FOR PRODUCTION USAGE!!!

(Redmine issue: #3734,#3361)