Various changes.

- Set loglevel for the requests module to WARNING.
- Performance boost by getting the content of the destination directory in the
  beginning and checking each potential downloaded file against that list
  instead of running os.path.exists() for every file.
- Fixed bug in redditdownload.py:download_from_url() that raises an
  AttributeError when the extension and the MIME type of the file diverge.
- Overhaul of all source files according to output of pep8
redditdownload.py uses requests now, extension changes.