Client now successfully connects to OPC foundation .net server.
Dependency on cryptography module is completely optional: you
can use python-opcua without cryptography if you only use #None policy.

Client and server should respect BufferSize values and
should not send chunks bigger than peer's BufferSize.
Also, added base class for SecurityPolicy. Only None policy
(no signing, no encryption) is implemented now.

implement filtering in server side find_servers

performance enhancement 2

uadiscover was broken after renaming of functions

Typo fix

print statement referenced root instead of objects

discovery implementation

this seems to comply with other servers

XML importer updates

Choose PolicyId from list of policies

Refactor command-line tools

In ActivateSession parameters, PolicyId must match the Id of needed
UserTokenPolicy of endpoint. Some servers ignore this Id, but some
(like OPC Foundation's .Net UA library) return an error if PolicyId is invalid.