Added support for file caching.
Refactor to have `ComicsAccount` cacheable on disk:
- changed the way the session gets saved in `clf`.
- use the disk cache in the web UI.
Fixed some bugs with the `cache` module.
Use percentages in the progress notification of the `CbzBuilder`.
Fixed some problems with how the cached account is obtained.
The `cache` class can only say whether is has an item with a given key, but it
won't say whether it's expired or not.
Front-end work:
- Added some CSS styling.
- Added support for download progress feedback.
- Better layout & presentation.
- Moved CSS file into the `css` directory.

--HG--
rename : static/style.css => static/css/style.css
Added `cache` to ignore file.
Front-end tweaks.
`cbz` module changes:
- Added `CbzLibrary` class to handle managed paths.
- Fixed a bug with the progress notification values.
- Download pages in a unique temp folder to support multiple downloads at once.
Wait before showing the download progress to account for server pre-process.
Server changes:
- Give the front-end info about already-downloaded issues.
- Error handling for the background downloader.
Fixed missing imports.
Added support for volume number and title. Added CDN-related functions.
Front-end improvements:
- Show full titles, including volume numbers/titles.
- Use the CDN to show resized covers.
- Fixed redirects for logging in.
- Better layouts.
Removed debug print.
Added support for config-set comicsbooks directory.
Added module for handling ComicRack metadata.
Added a DummyCache class for non-caching requests.
CBZ changes:
- Added metadata creation (using ComicRack format).
- Trap errors when cleaning up.
- Better file format.
Lots of changes in command-line & web CLF:
- Added progress information in command-line CLF.
- Code refactoring in web CLF.
- Reusing the issue title formatting code.
- Added some debug command-line feature.