Support for Arbitrary Object Keys

https://scireum.myjetbrains.com/youtrack/issue/MISC-11

Adds support for arbitrary object keys, including slashes
- This is achieved by URL-encoding object file names
- Simple keys without special characters remain the same
- Properties files and markers receive a new collision-free prefix $ (instead of __ninja_)
- The character $ will be encoded when used in object keys
- The required migration of the stored files is done on the fly, on the first access to the bucket
Centralises checking of bucket names and object keys
- Official rules for bucket names are now strictly enforced
Fixes and revises documentation comments