Add bumpversion support (and bump version to 4.0.2-dev)

Add bumpversion support to taurus. This allows us to keep a consistent version scheme.

With this, the release process would be as follows (it can be done manually or eventually be automated in the Continuous Delivery process):

• We start in develop, with an unreleased version (i.e., with "-dev" suffix). For example, say "4.0.2-dev"

• start release branch with:

  git checkout -b release-`bumpversion --dry-run --list release | grep new_version= | sed -r s,"^.*=",,` develop

  (the branch will be called "release-4.0.2")

• call bumpversion release (version in the release branch will now be "4.0.2")

• work on release branch and commit (if needed)

• edit CHANGELOG.md (if needed)

• merge release branch into master and tag with:

  git tag `bumpversion --dry-run --list release | grep current_version= | sed -r s,"^.*=",,`

• merge release branch into develop and bump patch version with bumpversion patch(version in develop will now be "4.0.3-dev")

• During the coming development process, any change that adds a new feature should call bumpversion minor and any change that produces backwards incompatibility should call bumpversion major. Bug fixes do not need to bump the version since we already did just after the release.