Fixed bug when percona config is nil

- bump version
- update CHANGELOG
Merge pull request #3 from steverice/fix-empty-percona-config

Fixed bug when percona config is nil
Support relative paths for defaults-file

- introduce mutator pattern for flag values
  - add make_path_absolute mutator
    - add tests
- refactor DEFAULT_FLAGS constant to PERCONA_FLAGS, which can contain additional metadata
  - add tests
- bump version to 0.0.3
  - add and update CHANGELOG
- update README