Allow for dirty installs from development

During testing it is required to install rwth.nb easily via pip on
remote servers. Therefore, do not be too strict on tagging every
installable version.

setup.py

@@ -35,9 +35,14 @@ def get_version():
         git_describe = f.readline()
         f.close()
 
+    # check whether
+    # 1. we have tagged our current version
+    # 2. it comes from the master branch
     version = git_describe
     split_describe = git_describe.split('-')
-    if (len(split_describe) > 1):
+
+    git_branch = subprocess.check_output(["git", "branch", "--show-current"])
+    if (len(split_describe) > 1 and git_branch == "master"):
         raise Exception("I do not want to build this package since you did not tag your current state of work.")
 
     return version