Use file size from thread JSON

chanarch.py (ChanThread):
  download_files():
    Read filesize from thread JSON and pass it to the Downloader, if
    available. This way, we can skip most files that are already
    downloaded without even connecting to the server.

chanarch.py (Downloader):
  download():
    Allow file size argument, in case the filesize is already known, and
    use it to skip downloading before even connecting to a server if it
    matches the downloaded file's size.
Use reStructuredText for documentation

AUTHORS:
  Renamed to AUTHORS.rst

AUTHORS.rst:
  Add reStructuredText markup

ChangeLog:
  Renamed to ChangeLog.rst

ChangeLog.rst:
  Add reStructuredText markup

README:
  Renamed to README.rst

README.rst:
  Add reStructuredText markup
  Add chanarch release page to the downloads section
Release Chanarch 0.1.1

- Use filesizes from the thread JSON to check if the file is downloaded
  to speed up file downloading
- Use reStructuredText for documentation
Chanarch 0.1.1

- Use filesizes from the thread JSON to check if the file is downloaded
  to speed up file downloading
- Use reStructuredText for documentation