Replace $USER with $USERNAME in globaleaks configuration file.

Following a discussion in #958, we decided to change the name of "$USER"
variable in /etc/default/globaleaks.
Use __future__.print_fuction in setup.py

A little step towards python3.
Skip the listing of directory files, if not present.

At the moment, cloning immediately glbackend and attempting to run tests will
result in a failure. This commit works around the error.

$ cd GLBackend && coverage run setup.py test && coveralls || true
Traceback (most recent call last):
  File "setup.py", line 64, in <module>
    list_files(os.path.join(glclient_path))),
  File "setup.py", line 46, in list_files
    for f in os.listdir(path):
OSError: [Errno 2] No such file or directory: 'glclient'

A better solution will be given in a couple of weeks, where both client and
backend will be integrated in the same repository globaleaks.