Add isort pre-commit hook

Sort import statement in python files

See https://github.com/PyCQA/isort

python_apps/airtime_analyzer/tests/metadata_analyzer_test.py

@@ -1,9 +1,11 @@
 from __future__ import print_function
+
 import datetime
-import mutagen
+
 import mock
-from nose.tools import *
+import mutagen
 from airtime_analyzer.metadata_analyzer import MetadataAnalyzer
+from nose.tools import *
 
 
 def setup():