Fix for Python 3.4 deprecation warnings while running tests

Running tests on Python 3.4 produces some deprecation warnings. This fixes those. Commit fc50509 is separated because it fixes a warning caused by `docutils`.

```
/home/dturgut/src/pelican/pelican/readers.py:234: DeprecationWarning: The value of convert_charrefs will become True in 3.5. You are encouraged to set the value explicitly.
  HTMLParser.__init__(self)

/home/dturgut/.virtualenvs/pelican-py3.4/lib/python3.4/site-packages/docutils/io.py:236: DeprecationWarning: 'U' mode is deprecated
  self.source = open(source_path, mode, **kwargs)

/home/dturgut/src/pelican/pelican/tools/pelican_import.py:132: DeprecationWarning: The value of convert_charrefs will become True in 3.5. You are encouraged to set the value explicitly.
  title = HTMLParser().unescape(item.title.contents[0])

/home/dturgut/src/pelican/pelican/tools/pelican_import.py:132: DeprecationWarning: The unescape method is deprecated and will be removed in 3.5, use html.unescape() instead.
  title = HTMLParser().unescape(item.title.contents[0])
```