V7SecurityManager and SubjectProvider modified to use VaadinSession to
hold current Subject.  Login fails, with a "Not an HTTP Session"
exception
Added VaadinSessionManager.  Loads app but fails with "cookies not
enabled" error
login works, but there are a couple of test failures from changing
security setup
All unit tests green, TestBench login test still to be written, although
login seems to work
 This is a combination of 2 commits.

Introduced use of VaadinSession to retain current Subject; modified
V7SecurityManager, DefaultShiroWebModule to be no longer web based.
Introduced VaadinSessionManager.  Fixes #180

Authentication seems to work correctly but there is still an issue with
reflecting the current login status via the user interface, see #181