Improve black hole support

Also scan the 'from' folder if Black hole is used together with another
downloader.
Update Synology and Pneumatic

As per black hole improvement
Increase check_snatched readability

- Reduce rested if statements
- Add more comments
Renamer rename to media
Provide IDs to check to all downloaders
Add backwards compatibility for the renamer API
Don't tag .ignore files
Dont remove historic data when doing a full scan.

Fixes #2572

Note that the dashboard already takes care of this and does it the right
way (keeping seeding and ignored releases).
Extend os.path.sep to all folder checks

Expands 50c5044fe8fa6f7ab624a72e8dba78453241b127
update contributing
Tabs to spaces
Improved checking

Fixes #2539 ?
Add another check
nzbmegasearch returns redirected url. fix #2597
Convert windows path to *nix path in sp

Fixes #2594

Note that os.path.normath converts '/' to '\\' on windows machines, but
unfortunately not the other way around...
Merge pull request #2545 from mano3m/develop_downloaders

Downloader and renamer improvements