Remove relationship between Work and Team, and Add team_id to Credit.

Just relationship between `Work` and `Team` was not sufficient to find members of the team who had been involved in making the `Work` at that time because members of team could always vary. So `team_id` is appended to columns in `Credit`. In this case, the relationship is redundant and therefore removed.

The below image is a diagram for the schema so far.
![model-0910](https://cloud.githubusercontent.com/assets/8075795/4217423/90def77e-38e7-11e4-824c-07ccd32aeb14.PNG)