This change allows for parallel downloads of HTML pages upto a maximum
number of concurrent HTTP requests which is more efficient than
before where all HTTP requests are likely to begin at the same time.
- Prevent downloading images with base64 strings as the source
- Add escaping of quotation characters in the serializer
- Disable redirects when downloading images which fails on multiple sites
- Remove invalid characters for making the epub export file name
- Fix version number in release
