Clean up client's security policy API

Also, move all crypto functions to uacrypto.

Added set_security() and set_security_string() methods to Client.
Removed duplicate server_certificate field, renamed
client_certificate and private_key to user_certificate and user_private_key.

pypy3 on travis

XML importer typo fix

Sync with master

fix deadlock

if two thread using internal seesion to create moniter items and change data value concurently, MonitoredItemService.create_monitored_items & AddressSpace.set_attribute_value may deadlock.

PEP-8 cleanup

tests for command lines

add shadow copy to Buffer

Set default security policy