Config loaders

Created by: stanislaw47

Hello Taurus community This PR provides what I wanted in Taurus for long time: ability for load configuration for TaurusGUI from JSON.

I created abstraction for config source, be it a Python module or XML/JSON file. In that way, adding support for any other config file format, like TOML or YAML should be trivial. TaurusGUI now loads configuration by accessing fields of *ConfigLoader class. Whole interface of *ConfigLoader class is defined in AbstractConfigLoader by using Python abc module.

All discovery logic happens in taurus.qt.qtgui.taurusgui.config_loader.getLoader function. It matches loaders by file extensions. In case of directory or non-existing path, it tries to use PyConfigLoader. In case of using entry-points, logic has to be moved elsewhere but for the moment I do not know how to achieve it, especially when taking into account possibility to pass directory and Python module path.

Regarding abc.ABC class for Python 2 and 3 - I know that Taurus uses future package for compatibility. However, I did not found anything there about providing abc.ABC so I created my own compatibility from what I found on StackOverflow.

Things to do:

• test for loading configuration from different sources
• backward compatibility tests
• entry-point for registering custom config loaders