Add clean-gopath target in Makefile

The clean-gopath will clean the packages in gopath. You can cleanly
build the server, afterwards, with the latest version of the dependencies
packages.
Fix bug in session.Session.GetUser()

It didn't return anything. It now returns the User field of session.
Adapt restit API changes

Added Reset methods to response structs used in restit tests.
Implemented better status codes

Use better status code response in all REST interfaces.

Added a method in PatRestHelper to count results for the purpose
to determine if there is no result in retrieve (to return 404).