update core.md

insert class description
Update core.md
Update core.md
Update core.md
Update index.md
add class and bundle descriptions
Update index.md
Rename deneral.md to general.md
Update core.md
Merge pull request #20 from muhasjo/UpdateWiki

Update wiki
fix: fix pubDate in rss
Create set/flush/add methods for model relations
Merge pull request #21 from funivan/master

Add model relations methods (add/flush/set)