started netstatus refactoring
netstatus refactoring
finished up NetStatus refactoring
merging ServerDescriptor with demo/master; fixed merged conflicts
Merge branch 'demo/master' into demo/connection_fsm

This merged Nik's recent refactor of the netstatus package.
Filled in details of `connection/handshake/v3.py`

Also made some unit tests and fixed some bugs in the various handshake
classes.
Fixed bugs; revised `test_circuit_handshakes.py`.