TaskFilter: Use empty string for None searches
Merge pull request #8 from tbabej/develop

TaskFilter: Use empty string for None searches
Update docs with new supported versions
Bump version for 0.5.0 release
Merge branch 'release/0.5.0' into develop
Task: Perform done and deleted operations only when necessary
Use uuid fields intead of ids where possible

IDs can get non-unique with completed tasks, which makes modifications
or deletions of particular completed tasks impossible.
Task: Load task uuid when saving a new task

A new task can not refresh its data since it has not set uuid/id.
We need to parse the output of the 'task add' command.
Task: List all non-readable fields as modified when creating new task
Task: Add NotSaved exception
Task: Allow setting attributes via keyword arguments
tests: Do not use TW directly to create tasks
tests: Add tests to filter by attributes and their empty values
Task: Raise exceptions via Task class reference
tests: Add basic Task tests
TaskWarrior: Detect task version
Task: Add workaround for task bug in older versions, which eats up a part of description
tests: Make tests Python 2.6 compatible, avoid using assertRaises as context manager