Backport: replacing print() by logging api

Conflicts:
	src/paperwork/backend/docimport.py
	src/paperwork/backend/docsearch.py
	src/paperwork/backend/pdf/page.py
	src/paperwork/frontend/mainwindow.py

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Backport: displaying to much traceback

it's not necessary to display the whole traceback through logging.exception for every exception caught

Conflicts:
	src/paperwork/backend/docsearch.py

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
AUTHORS: Add Francois Schmidts (jaesivsm)

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Backport: Logging: Adjust the logging level on some messages

Signed-off-by: Jerome Flesch <jflesch@gmail.com>

Conflicts:
	src/paperwork/backend/pdf/page.py
Logging: make logging level changeable using environment variable

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
main(): remove function check_module_versions(): It's obsolete (replaced by setuptools)

Signed-off-by: Jerome Flesch <jflesch@wyplay.com>
Documentation: Add informations on how to write a helpful bug report + how to increase the verbosity level

Signed-off-by: Jerome Flesch <jflesch@gmail.com>
Merge branch 'testing' into unstable

Conflicts:
	src/paperwork/backend/docsearch.py
	src/paperwork/frontend/mainwindow.py