Add delete_redditor functionality for AuthenticatedReddit class (#240)

AuthenticatedReddit now has a method delete_redditor which deletes the currently logged-in redditor. 

Works when tested manually:

    >>> import praw
    >>> r = praw.Reddit('dsf', site_name='local')
    >>> r.create_redditor('d_schrute', 'beets')
    {u'errors': [], u'data': {u'modhash': u'd_schrute', u'cookie':u'1748,2014-05-04T14:28:05,81e7d58598850870c330fd35eb2b0a34d3102dfa'}}
    >>> r.login('d_schrute', 'beets')
    >>> r.user.name
    'd_schrute'
    >>> r.delete_redditor('beets')
    {u'errors': []}
    >>> r.clear_authentication()
    >>> r.login('d_schrute', 'beets')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "praw/__init__.py", line 1212, in login
        self.request_json(self.config['login'], data=data)
      File "praw/decorators.py", line 177, in wrapped
        raise error_list[0]
    praw.errors.InvalidUserPass: `invalid password` on field `passwd`
    >>> r.create_redditor('d_schrute', 'beets')
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
      File "praw/decorators.py", line 225, in wrapped
        return function(obj, *args, **kwargs)
      File "praw/__init__.py", line 664, in create_redditor
        return self.request_json(self.config['register'], data=data)
      File "praw/decorators.py", line 177, in wrapped
        raise error_list[0]
    praw.errors.APIException: (USERNAME_TAKEN_DEL) `that username is taken by a deleted  account` on field `user`


And the test suites seem to work, if I'm doing it right:

    >>> suite = unittest.TestSuite()
    >>> suite.addTest(praw.tests.LocalOnlyTest('test_delete_existing_redditor'))
    >>> suite.addTest(praw.tests.LocalOnlyTest('test_create_deleted_redditor'))
    >>> suite.addTest(praw.tests.LocalOnlyTest('test_deleted_redditor_login'))
    >>> unittest.TextTestRunner().run(suite)
    Passing local only test: LocalOnlyTest.test_delete_existing_redditor
    .Passing local only test: LocalOnlyTest.test_create_deleted_redditor
    .Passing local only test: LocalOnlyTest.test_deleted_redditor_login
    .
    ----------------------------------------------------------------------
    Ran 3 tests in 12.830s
    
    OK
    <unittest.runner.TextTestResult run=3 errors=0 failures=0>