Add config for Travis and CodeClimate

- updated README with supported versions of ActiveRecord/Ruby
Merge pull request #1 from steverice/add-travis

Add config for Travis and CodeClimate
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
  - update CHANGELOG
- update README