Add UploadCare.file_group and testing of regex
Split tests.py
Add container methods to FileGroup
Add ``FileGroup.store`` method
Optimize indexing of group's files
Add models and forms FileGroupField
Fix group regex
Move file, file_group validations into file module
Add ``file_cdn_urls`` property

It returns CDN urls of all files from group without API requesting.
Merge branch 'master' into file-group

Conflicts:
	tests/test_file.py
Replace load_file_from_cache by File.construct_from
Merge branch 'master' into file-group
Add FileGroup documentation
Make api_uri, api_storage_uri properties private
Remove @property from methods that might do API requests
Add some minor FileGroup documentation's updates
Rename FileGroup.group_id to FileGroup.uuid
Add FileGroupField description to django-widget.rst
Add ImageGroupField and some documentation
Add handling of deleted group files