Codestyle improvements
Some improvements:

  - Improved codestyle;
  - Better search existing tasks (now it search by id, subject and
  project id attrs);
  - Improved nesting saving.
A bunch of updates:

  - Removed useless transaction in import;
  - Added units of using resource to export (100% for assignee);
  - Enabled predecessors export;
  - Reworked collecting of exported versions;
  - Added milestones import (converts to redmine versions);
  - Added support relations between tasks and milestones;
  - Added gitignore file.