Imports: Add isort

- Add info for isort in development setup
- Add default config for isort behaviour
  - Ignore a number of files and folders
Imports: setup.py
Imports: sort tests/
Imports: sort tools/
Imports: sort translate/
Imports: sort translate/convert/
Imports: sort translate/convert/test_*
Imports: sort translate/filters/
Imports: sort translate/lang/
Imports: sort translate/misc/
Imports: sort translate/search/
Imports: sort translate/services/
Imports: sort translate/tools/

No sorting of test_* or commandline scripts
Imports: sort translate/tools/test_*
Imports: sort translate/storage/
Imports: sort translate/storage/test_*
Imports: sort translate/storage/xml_extract/
Imports: sort translate/storage/placeables/
Imports: sort translate/storage/versioncontrol/