Fixed an issue when zipped resources couldn't have been loaded

`LookupError` is raised when I'm trying to load a zipped corpus (full error message at the bottom).

The zip is there and successfully found — error happens when `ZipFilePathPointer` is accessing a path within the file. Code on line 452

    # Normalize the entry string, it should be absolute:
    entry = normalize_resource_name(entry, False, '/').lstrip('/')

returns this:

    >>> data.normalize_resource_name('/dir/file', allow_relative=False, '/').lstrip('/')
    'C:/dir/file'

and of course, such an entry cannot be found within a zipfile. In case of a zipfile, `allow_relative` should be set to `True`. This way `os.path.abspath()` function will not be called, and we get a proper path within the archive:

    >>> data.normalize_resource_name('/dir/file', True, '/').lstrip('/')
    'dir/file'

---
    >>> reuters
    <CategorizedPlaintextCorpusReader in '.../corpora/reuters' (not loaded yet)>
    >>> reuters.fileids()
    Traceback (most recent call last):
      File "C:\Python34\lib\site-packages\nltk\corpus\util.py", line 64, in __load
        try: root = nltk.data.find('corpora/%s' % zip_name)
      File "C:\Python34\lib\site-packages\nltk\data.py", line 618, in find
        raise LookupError(resource_not_found)
    LookupError:
    **********************************************************************
      Resource 'corpora/reuters.zip/reuters/' not found.  Please use
      the NLTK Downloader to obtain the resource:  >>> nltk.download()
      Searched in:
        - 'C:\\Users\\Andrey/nltk_data'
        - 'C:\\nltk_data'
        - 'D:\\nltk_data'
        - 'E:\\nltk_data'
        - 'C:\\Python34\\nltk_data'
        - 'C:\\Python34\\lib\\nltk_data'
        - 'C:\\Users\\Andrey\\AppData\\Roaming\\nltk_data'
    **********************************************************************

    During handling of the above exception, another exception occurred:

    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "C:\Python34\lib\site-packages\nltk\corpus\util.py", line 100, in __getattr__
        self.__load()
      File "C:\Python34\lib\site-packages\nltk\corpus\util.py", line 65, in __load
        except LookupError: raise e
      File "C:\Python34\lib\site-packages\nltk\corpus\util.py", line 62, in __load
        root = nltk.data.find('corpora/%s' % self.__name)
      File "C:\Python34\lib\site-packages\nltk\data.py", line 618, in find
        raise LookupError(resource_not_found)
    LookupError:
    **********************************************************************
      Resource 'corpora/reuters' not found.  Please use the NLTK
      Downloader to obtain the resource:  >>> nltk.download()
      Searched in:
        - 'C:\\Users\\Andrey/nltk_data'
        - 'C:\\nltk_data'
        - 'D:\\nltk_data'
        - 'E:\\nltk_data'
        - 'C:\\Python34\\nltk_data'
        - 'C:\\Python34\\lib\\nltk_data'
        - 'C:\\Users\\Andrey\\AppData\\Roaming\\nltk_data'
    **********************************************************************