Add sync methods

[Ilyazyk]

Can we please have added two Client's sync methods:
asyncua\sync.py

class Client:
@syncmethod
def write_values(self):
pass

@syncmethod
def read_values(self):
    pass

[swamper123]

You can provide a PR for that.

[Ilyazyk]

I just can't push new branch from my Eclipse (push not permitted) nether by https nor ssh. Any tips are welcome. Or just create the PR please.

[AndreasHeine]

fork, create branch, comit and pr ?

[Ilyazyk]

Finally, I've created PR "write_values & read_values #541". Hope it's done correctly.