Changed behavoiur for duplicates

Till now, if we download certain files (like those attached to the
conversation) and we already have the file with the same name, number in
format '%03d' was added just before extension. That way there could be
possibility, that the very same file will be downloaded and stored
multiple times, like:

file.png
file.001.png
file.002.png
...

This commit prevents that by adding comparison between files we already
have and file which is downloaded from slack. Adding another file with
additional number will only have place when stored file and downloaded
have different content.

slack_backup/utils.py

@@ -4,6 +4,8 @@ Some utils functions. Jsut to not copypaste the code around
 import errno
 import os
 import logging
+import tempfile
+import hashlib
 
 
 def makedirs(path):
@@ -19,3 +21,24 @@ def makedirs(path):
                 logging.error("Cannot create `%s'. There is some file on the "
                               "way; cannot proceed.", path)
                 raise
+
+
+def get_temp_name():
+    """Return temporary file name"""
+    fdesc, fname = tempfile.mkstemp()
+    os.close(fdesc)
+    return fname
+
+
+def same_files(file1, file2):
+    """
+    Compare files by calculating hash for each of them. Return True if hash is
+    identical, False otherwise
+    """
+    with open(file1, 'rb') as fobj:
+        hash1 = hashlib.sha256(fobj.read())
+
+    with open(file2, 'rb') as fobj:
+        hash2 = hashlib.sha256(fobj.read())
+
+    return hash1.hexdigest() == hash2.hexdigest()