Add support for declaring component's scope

[saskodh]

Task

• implement @scope decorator
• add support for the following scopes:
  • singleton (default)
  • prototype - new instance is created each time the component is referenced
  • request - new instance is created for each request (after zone.js is integrated)
  • session - new instance is created for each different user session (can be complicated to implement, maybe for later)
  • custom scope - enable devs to implement their own custom scopes (check the complexity for the priority)