Update Upload lib docs
Adjustments to the previous commit
Uniformize slash_item() with item()
Fix #2558
Fix #2560
Form helper: refactor form_open() and _attributes_to_string()
More logical order
Merge pull request #2565 from vlakoff/develop

Form helper: refactor form_open() and _attributes_to_string()
Merge pull request #2554 from vlakoff/develop-3

config->item() now returns NULL instead of FALSE when the required item doesn't exist.
Complete 3.0 upgrade notes about NULL returned on missing items
Form helper _attributes_to_string() micro-optimization

As $attributes should be most of the times an array, let's save an is_string() call.
Polishing Form helper
Change spaces to tabs
Fix #2568
Merge pull request #2567 from vlakoff/develop-2

Polishing Form helper
Cache Log Error Levels
The log messages in the cache drvier's is_supported calls are more suited for the debug level.
Updated Changelog Cache Log Level
- Updated the changelog to include the log level updates in the cache drivers.
Merge pull request #2569 from refringe/develop

Cache Log Error Levels
Optimize CI_Session::__construct() routines and make driver validity check stricter
Add windows filename rule as an option for upload files