Switch to different Git workflow

Up to now all development happened on the `dev` branch whereas `master` was
always 'deployable'. Releasing a new version included merging it into `master`,
tagging it there and then returning back. From now on everything will
just happen on the master branch with stable releases marked by git tags.
This significantly reduces the overhead involved in releasing new versions.

README.rst

@@ -262,11 +262,11 @@ v0.1 (*2013-03-14*)
 
 - Initial Release
 
-.. |Build Status| image:: http://img.shields.io/travis/msiemens/PyGitUp/dev.svg?style=flat-square
+.. |Build Status| image:: http://img.shields.io/travis/msiemens/PyGitUp/master.svg?style=flat-square
    :target: https://travis-ci.org/msiemens/PyGitUp
 
-.. |Coverage Status| image:: http://img.shields.io/coveralls/msiemens/PyGitUp/dev.svg?style=flat-square
-  :target: https://coveralls.io/r/msiemens/PyGitUp?branch=dev
+.. |Coverage Status| image:: http://img.shields.io/coveralls/msiemens/PyGitUp/master.svg?style=flat-square
+  :target: https://coveralls.io/r/msiemens/PyGitUp
 
 .. |Version| image:: http://img.shields.io/pypi/v/git-up.svg?style=flat-square
   :target: https://pypi.python.org/pypi/git-up