Oschmod

**Summary**:

* Adds symbolic representation for mode changes (e.g., `+x`, `u+rwx`, `ugo-x`).

**Summary**:

* Fix issue with account lookup failing with error (`No mapping between account names and security IDs was done.`).

**Summary**:

* Add recursive version of `oschmod.set_mode()`: `oschmod.set_mode_recursive()`

**Summary**:

* Revise Windows to keep permissions an object already has for SYSTEM/NT AUTHORITY and don't include those permissions in mode calculation.
* Fix various minor cosmetic issues.

**Summary**:

* Initial release

**Commit Delta**: [Change from 0.2.4 release](https://github.com/YakDriver/oschmod/compare/0.2.4...0.3.0)