Adding some documentation in the Session class. Also testing the Id svn...

Adding some documentation in the Session class. Also testing the Id svn keyword, to see if it works. I plan to add a new phpdoc header to all files, including versioning with Id

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@197 44740490-163a-0410-bde0-09ae8108e29a