Refactor client code

Refactor client code to follow this structure:

    time-booker/ 
      |- client/                
      |  |- src/                 
      |  |  |- adapters/
      |  |  |- components/
      |  |  |- controllers/
      |  |- assets/
      |  |- vendor/
         etc.
- [x] Add more gulp task
- [x] Add git pre-commit hook