[Issue #3] Users can store URLs into boxes

  - Model, Controller and views for Links
  - Added relationships between models link-box-user
  - Initial Specs for basic behaviours
[Issue #3] More specs

  - Added more specs checking destroy dependence between boxes and links
  - Added more specs checking actions that are not allowed without a valid session
Merge pull request #8 from insignia4u/Issue#3-Links

[Issue#3] Users can store URLs into boxes