…read/writeable for them

Prevents
1. information disclosure
2. unpickling of untrusted pickle files resulting in code execution
vulnerabilities

Execute as user `nobody`:
```
$ python3
>>> with open('/dev/shm/sm_foo', 'wb') as fd:
...  fd.write(b'\x80\x03csubprocess\ncall\nq\x00X\n\x00\x00\x00/bin/touchq\x01X\x0b\x00\x00\x00/tmp/hackedq\x02\x86q\x03\x85q\x04Rq\x05.')
...
66
$ ls -l '/dev/shm/sm_foo'
-rw-r--r-- 1 nobody nogroup 66 Okt 21 18:42 /dev/shm/sm_foo
```

Then execute a new process as any user (e.g. root):

```
$ python3
>>> import shared_memory_dict
>>> f = shared_memory_dict.SharedMemoryDict('foo', 500)
>>> f
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/fbest/git/shared-memory-dict/shared_memory_dict/dict.py", line 115, in __repr__
    return repr(self._read_memory())
  File "/home/fbest/git/shared-memory-dict/shared_memory_dict/dict.py", line 169, in _read_memory
    db = {key: self._unmap_value(key, value) for key, value in db.items()}
AttributeError: 'int' object has no attribute 'items'

$ ls -l /tmp/hacked
-rw-r--r-- 1 root root 0 Okt 21 18:45 /tmp/hacked
```

The command /bin/touch /tmp/hacked has been executed as root.

Fixes luizalabs#33