[Sprint #5] developing basic APIs

#### Summary
- basic retrieve APIs ( how about Create / Update / Destroy ? it has been intentionally excluded from current API because we don't have any model level validation, currently using django's manager level validation. )
- APIs for search : will be used for auto-complete and search feature in client development.

#### Developed APIs
- `/api/users/`
- `/api/users/?search={query}`
- `/api/projects/`
- `/api/projects/?search={query}`
- `/api/teams/`
- `/api/teams/?search={query}`
- `/api/teams/<teamname>/users/`
- `/api/teams/<teamname>/users/?search={query}`
- `/api/teams/<teamname>/projects/`
- `/api/teams/<teamname>/projects/?search={query}`