Add include parameter for posts API

closes #2609
- added include parameter to api.posts.*
- changed toJSON to omit objects that are not included
- added include parameter to admin
- added include parameter to frontend.js
- updated tests
- removed duplicate code from posts model

**Known Issue:** It is not possible to attach a tag using an ID.
Merge pull request #2646 from sebgie/issue#2609

Add include parameter for posts API