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.

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## Remains to be done:
- [x] Update the version number in the Config module
- [x] Write the release notes into the official documentation
- [x] Update the release date in the NEWS file