File metadata: Add file metadata app

Add the file metadata app. This app uses binary wrappers called drivers
to extract properties from the file of documents. The default driver
uses the exiftool to extract the EXIF record from JPEG images.
The exiftool can also extra some properties from other files like
PDFs, office files and sound file.

Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>

mayan/apps/file_metadata/tasks.py

@@ -0,0 +1,24 @@
+from __future__ import unicode_literals
+
+import logging
+
+from django.apps import apps
+
+from mayan.celery import app
+
+from .classes import FileMetadataDriver
+
+logger = logging.getLogger(__name__)
+
+
+@app.task(ignore_result=True)
+def task_process_document_version(document_version_id):
+    DocumentVersion = apps.get_model(
+        app_label='documents', model_name='DocumentVersion'
+    )
+
+    document_version = DocumentVersion.objects.get(pk=document_version_id)
+
+    FileMetadataDriver.process_document_version(
+        document_version=document_version
+    )