Star projects

Star project:

     POST /projects/<project id>/star

Unstar project:
     POST /projects/<project id>/unstar

List project fans:

     GET /projects/<project id>/fans
     GET /projects/<project id>/fans/<user id>

List user starred projects:

     GET /users/<user id>/starred
     GET /users/<user id>/starred/<project id>

Also a "star" field with the stars count of a project is included in the
project detail and list responses.