Cleanup: Remove code for supporting Python versions older than 2.5

There is still some code in the codebase that provides substitutes for code
present on newer Python versions, but those cleanups are left for the future.
Drop support for Python 2.5

There is still some code in the codebase that provides substitutes for code
present on newer Python versions, but those cleanups are left for the future.
Styleguide: Alter one-line docstring summary recommendations to match PEP 257
Cleanups: convert to sections
Docs: Add import order on styleguide
Docs: Add notes on from ... imports order and long imports in styleguide
Docs: Add section on properties to styleguide
Docs: Add docs about Android translation format
Add comments exporting support in po2tmx. See also bugzilla #3100.