Implement request #301: Create users when using external auth login

After a successful login with imap, smb or ftp, the corresponding
user is created in an own database table.
This is needed to make e.g. sharing files with this users work.

The table may be used by several different external user authentication
services.

See the discussion in https://github.com/owncloud/apps/pull/1579
for details on the implementation design.
Fix code according to blizzz' notes
remove superfluous class path definition
fix notnull settings
add missing backticks to database fields
Fix coding style according to owncloud/build/phpcs.xml
Fix typos, owncloud -> ownCloud
use executeAudited instead of prepare+execute
Merge pull request #1579 from cweiske/auto-create-user

Implement request #301: Create users when using external auth login