Added support for utf-encoded image paths

Running PlexConnect and browsing my swedish channels, e.g. SVT Play, rendered the following error in the log:

12:29:57 XMLConverter: XML_ExpandLine - Error in {{IMAGEURL(thumb::384)}}
Traceback (most recent call last):
  File "/Users/mattias/projects/sources/PlexConnect/XMLConverter.py", line 561, in XML_ExpandLine
    res = getattr(g_CommandCollection, 'ATTRIB_'+cmd)(src, srcXML, param)
  File "/Users/mattias/projects/sources/PlexConnect/XMLConverter.py", line 909, in ATTRIB_IMAGEURL
    res = PlexAPI.getTranscodeImagePath(key, AuthToken, self.path[srcXML], width, height)
  File "/Users/mattias/projects/sources/PlexConnect/PlexAPI.py", line 624, in getTranscodeImagePath
    path = '/photo/:/transcode/%s/?width=%s&height=%s&url=' % (quote_plus(path), width, height) + quote_plus(path)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib.py", line 1289, in quote_plus
    return quote(s, safe)
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/urllib.py", line 1282, in quote
    return ''.join(map(quoter, s))
KeyError: u'\xe4'

This fix corrected it, as there were utf8-encoded characters in the path (åäö etc)