Refactor Readable.readable_parent to consider all ancestors

[meacuna]

Fixes #82

[coveralls]

Coverage remained the same at 100.0% when pulling 1953ed5 on meacuna:feature/support_multi_level_inheritance into 77a17f6 on ledermann:master.

[ledermann]

Great improvement! Could you a test to verify that the change works?

[coveralls]

Coverage remained the same at 100.0% when pulling e1ab77c on meacuna:feature/support_multi_level_inheritance into 77a17f6 on ledermann:master.

[meacuna]

@ledermann I pushed some modifications to the test suite you had to include the changes

[coveralls]

Coverage decreased (-35.0%) to 65.049% when pulling ca2cdb9 on meacuna:feature/support_multi_level_inheritance into 77a17f6 on ledermann:master.

[ledermann]

@meacuna Merged, thank you very much!