[#749] Add RSR version header middleware

The middleware adds a response header of the format:

X-RSR-Verision: "Tag:<tag name> Commit:<commit hash> Branch:<git branch name>"
[#749] Merge branch 'develop' of github.com:akvo/akvo-rsr into feature/749_rsr_version_header
Merge pull request #771 from akvo/feature/749_rsr_version_header

Code checked