Minor: Switched master branch from 3.0.x to 3.1.x

- updated README to reflect changes
Fix: Some typos in README file
Some updates for the SilverStripe 3.0.x branch
Moved some settings from _config.php to _config/settings.yml
Update README.md
Update README.md
Added option to sort images by EXIF creation date

- can be activated via _config/settings.yml IMAGE_SORT_OPTION: 4
- thanks to Joop53 for the idea
Removed unsupported setting from 3.0.x branch
Updated comment blocks
Fixed possible timeout issues when updating EXIF dates

- limited EXIF database updates to image objects assigned to page saved
- fix possible timeout issues when dealing with lots of image objects