implement extract and generate

ran tests successfully with
$cd i18n; python -m unittest test

conf/locale/en/LC_MESSAGES/messages.po

+# empty

i18n/test/__init__.py

+from test_extract import TestExtract

i18n/test/test_extract.py

+import os
+from unittest import TestCase
+from datetime import datetime
+import polib
+
+import extract
+from execute import SOURCE_MSGS_DIR
+
+class TestExtract(TestCase):
+    """
+    Tests functionality of i18n/extract.py
+    """
+    generated_files = ('django.po', 'djangojs.po', 'mako.po')
+
+    def setUp(self):
+        self.start_time = datetime.now()
+        extract.main()
+
+    def get_files (self):
+        """
+        This is a generator.
+        Returns the fully expanded filenames for all extracted files
+        Fails assertion if one of the files doesn't exist.
+        """
+        for filename in self.generated_files:
+            path = os.path.join(SOURCE_MSGS_DIR, filename)
+            exists = os.path.exists(path)
+            self.assertTrue(exists, msg='Missing file: %s' % filename)
+            if exists:
+                yield path
+
+    def test_files(self):
+        """
+        Asserts that each auto-generated file has been modified since 'extract' was launched.
+        Intended to show that the file has been touched by 'extract'.
+        """
+
+        for path in self.get_files():
+            self.assertTrue(datetime.fromtimestamp(os.path.getmtime(path)) > self.start_time,
+                            msg='File not recently modified: %s' % os.path.basename(path))
+
+    def test_is_keystring(self):
+        """
+        Verifies is_keystring predicate
+        """
+        entry1 = polib.POEntry()
+        entry2 = polib.POEntry()
+        entry1.msgid = "_.lms.admin.warning.keystring"
+        entry2.msgid = "This is not a keystring"
+        self.assertTrue(extract.is_key_string(entry1.msgid))
+        self.assertFalse(extract.is_key_string(entry2.msgid))
+    
+    def test_headers(self):
+        """Verify all headers have been modified"""
+        for path in self.get_files():
+            po = polib.pofile(path)
+            header = po.header
+            self.assertEqual(header.find('edX translation file'), 0,
+                             msg='Missing header in %s:\n"%s"' % \
+                             (os.path.basename(path), header))
+
+    def test_metadata(self):
+        """Verify all metadata has been modified"""
+        for path in self.get_files():    
+            po = polib.pofile(path)
+            metadata = po.metadata
+            value = metadata['Report-Msgid-Bugs-To']
+            expected = 'translation_team@edx.org'
+            self.assertEquals(expected, value)