Data structure change

In order to fit in a RESTful paradigm, it is hard to handle skills
and interests in the previous data structure design. It is much
easier to identify a user-skill or user-interest relation with
a relation id. And it is more expansible that way.
Separate users related api interface to users.go

Separate code about "api.v1/users" interface to users.go
Rename tests/user.go to tests/users.go

Align name with API interface
Implemented userSkills interface

Implemented both the API and the integration test.
Implemented userInterests interface

Implemented both the API and the integration test.
Merge pull request #2 from opensourcehk/yookoala_golang_skill

Yookoala golang skill