Simplify branch management

[olafmersmann]

Currently we have a convoluted workflow with a master, release (really a staging/pre-release branch) and development branch in addition to feature branches. I would be in favor of dropping the development and release branches and adopting a "live at head" development model.
All (major) changes would be developed in a separate feature branch (as we already do) and merged into master once all tests pass via squashing pull request. Squashing the commits leads to a cleaner history for the master branch but isn't a must. Official releases would still be marked by tags.

Am I missing some advantages of the current model compared to the simpler master-only model?