Version 1.3. list all uploads, category filter...

- Added the "list" request type to get a list of all public uploads
(files actually uploaded, not created with our generators)
- The search request now accepts a category filter :
...&req=search&category=math   for example.
- gz compression is now always enabled, so the manual flag for that is
gone.
- Fix some XML encoding issue (unescaped characters that messes up
SimpleXMLElement...)
- Better archive visibility handling (public/unlisted/private).
(Actually this was done some time ago, just not committed...)
- 'prettyjson' output format added.
- No change on the auth. side, although Apache has been updated since
last time, so HTTP auth could be worked on...
- Misc. README changes.
updated changelog for version 1.3