Added Tag model to backend

Updated relations between models
Implemented POST to /streams/:id/tags to assign tags

Also added embeded tags to streams
[Closes #19]
[Closes #21]
Added ability to remove tags from a stream

[Closes #20]
Merge
Merge branch 'master' into dev-tags

* master:
Merge pull request #26 from peepoltv/dev-tags

Added TAGs feature to API