Review unicode support

[SevereOverfl0w]

Unicode support was added somewhat hastily. I pretty much just shoved a b in front of all the strings, and switched StringIO for BytesIO.

• I'm not entirely sure that StringIO is even used anymore. The import may be removable
• For some reason, StringIO can be imported as either from StringIO import StringIO or from io import StringIO, what are their differences? does something similar apply to BytesIO?
• This code

  nrepl-python-client/nrepl/bencode.py

  Lines 106 to 109 in a97ba25

  	# x = x.encode("UTF-8")
  	# TODO revisit encodings, this is surely not right. Python
  	# (2.x, anyway) conflates bytes and strings, but 3.x does not...
  	out.write(str(len(x.encode('utf-8'))).encode('utf-8'))

  the comment probably doesn't need to be there anymore and it should be split up to ensure that it's understood that it's doing two things:
• Getting the length of the string in bytes
• Turning that length into a utf-8 encoded string

There's probably more refactoring to look into, but this is what comes to mind.