Python 3 compatibility issues

[cclauss]

Solutions in #105

$ flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics

./elfinder/connector.py:429:33: E999 SyntaxError: invalid syntax
                except Exception, e:
                                ^
./elfinder/utils/volumes.py:30:27: F821 undefined name 'basestring'
    if isinstance(class_, basestring) and class_:
                          ^
./elfinder/volumes/storage.py:72:53: F821 undefined name 'basestring'
                if isinstance(opts['storageClass'], basestring):
                                                    ^
./elfinder/volumes/filesystem.py:29:40: E999 SyntaxError: invalid token
        self._options['dirMode']  = 0755 #new dirs mode
                                       ^
./elfinder/volumes/base.py:199:37: F821 undefined name 'unicode'
        self._root = self._normpath(unicode(self._options['path']))
                                    ^
./elfinder/tests/volumes.py:38:56: F821 undefined name 'basestring'
        self.assertEqual(isinstance(self.default_path, basestring), True)
                                                       ^
2     E999 SyntaxError: invalid syntax
4     F821 undefined name 'basestring'
6