Issue 400: add documentation versioning

[jkotan]

It implements documentation versioning mentioned in #400, i.e.

• each documentation version corresponds to a separate branch e.g. docs_stable, docs_v0.4.1, docs_v0.3.3, ...
• pushing to the docs_* banches updates the corresponding html directory of the gh-pages branch i.e. pushing doc_stable updates the ./stable/ directory of the gh-pages branch
• pushing to the master banches updates the ./latest/ directory of the gh-pages branch
• the new index.html file redirects to ./stable/index.html
• documentation for VERSION with one '.' is stored as v:latest with (latest) label
• links to various versions of the documentation have been added to the index.rst.in file (it need to be updated to add new versions - probable we don't want to have a links for all possible h5cpp versions)
• when the release tag is pushed (the branch_name is equal to the v*.. tag_name) the new docs_v*.. branch is created and content of master is merged to docs_stable
• documentation bug fixes can be applied to the corresponding docs_* branches.

Before merging the PR we need to move content the gh-pages branch its new latest subdirectory.
We also need to create the docs_stable branch and other docs branches for older releases.