Proposal for accessing populated relations

fixes #842

- allows checking whether a relation has been populated
- getting a list of relation names that have been populated
- getting all populated relation data

todo:

- [] add phpdoc
made relation names in AR case sensitive
forgot to remove strtolower from __get and __unset
php-doc

[ci skip]
Merge pull request #1125 from cebe/842-relation-data

Proposal for accessing populated relations