Moving away from GitFlow

[augustofg]

With the v1.6.0 release out we decided to stop using GitFlow as it seems to be causing unnecessary friction without bringing any advantage over a simpler flow using the 'master' branch as the main development branch. The new development flow is described bellow:

• master is the main development branch;
• All features should start as a separate branch from master, and should be rebased on it before merging using --ff-only to keep the history linear;
• Each new release will be tagged on the master branch.

If we find necessary to separate bugfixes from new features to support older releases, them a new branch for the release should be created, and the necessary patches should be applied on top of it.