Release 0.4.0

This pull request is dedicated to releasing the version 0.4.0.

This version is a minor but important improvement. It contains:

## Improvement
- [x] #88: New host module and fixed memory collector for OS X 10.9 Mavericks.

---
## Remains to be done:
- [ ] Update the version number in the Config module
- [ ] Write the release note into the official documentation
- [ ] Update the release date in the NEWS file