fix python3 set_clipboard error

[KazuCocoa]

Python 3 requires encoding in bytes. Python 2 doesn't.
close #265

Python 3

>>> driver = webdriver.Remote('http://localhost:4723/wd/hub', desired_caps)
>>> driver.set_clipboard_text("hello")
>>> b = driver.get_clipboard()
>>> b.decode()
'hello'
>>> driver.set_clipboard_text("ありがとう")
>>> b = driver.get_clipboard()
>>> b.decode()
'ありがとう'

Python 2

>>> driver = webdriver.Remote('http://localhost:4723/wd/hub', desired_caps)
>>> driver.set_clipboard_text("ありがとう")
>>> b = driver.get_clipboard()
>>> print(unicode(b.decode('UTF-8')))
ありがとう
>>> driver.set_clipboard_text("hello")
>>> b = driver.get_clipboard()
>>> print(unicode(b.decode('UTF-8')))
hello

[mykola-mokhnach]

does it make sense to put this helper method into some separate helper module?

[KazuCocoa]

I thought so, but nothing method used bytes so far. I'd like to extract here in next time.