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/classes.py

@@ -0,0 +1,80 @@
+from __future__ import unicode_literals
+
+import logging
+
+from django.apps import apps
+
+from .events import event_file_metadata_document_version_finish
+from .exceptions import FileMetadataDriverError
+from .signals import post_document_version_file_metadata_processing
+
+logger = logging.getLogger(__name__)
+
+
+class FileMetadataDriver(object):
+    _registry = {}
+
+    @classmethod
+    def register(cls, mimetypes):
+        for mimetype in mimetypes:
+            cls._registry.setdefault(mimetype, []).append(cls)
+
+    @classmethod
+    def process_document_version(cls, document_version):
+        for driver_class in cls._registry.get(document_version.mimetype, ()):
+            try:
+                driver = driver_class()
+                driver.process(document_version=document_version)
+            except FileMetadataDriverError:
+                # If driver raises error, try next in the list
+                pass
+            else:
+                # If driver was successfull there is no need to try
+                # others in the list for this mimetype
+
+                event_file_metadata_document_version_finish.commit(
+                    action_object=document_version.document,
+                    target=document_version
+                )
+
+                post_document_version_file_metadata_processing.send(
+                    sender=document_version.__class__,
+                    instance=document_version
+                )
+                return
+
+    def process(self, document_version):
+        logger.info(
+            'Starting processing document version: %s', document_version
+        )
+
+        StoredDriver = apps.get_model(
+            app_label='file_metadata', model_name='StoredDriver'
+        )
+
+        driver_path = '.'.join([self.__module__, self.__class__.__name__])
+
+        driver, created = StoredDriver.objects.get_or_create(
+            driver_path=driver_path, defaults={
+                'internal_name': self.internal_name
+            }
+        )
+
+        driver.driver_entries.filter(
+            document_version=document_version
+        ).delete()
+
+        document_version_driver_entry = driver.driver_entries.create(
+            document_version=document_version
+        )
+
+        for key, value in self._process(document_version=document_version).items():
+            document_version_driver_entry.entries.create(
+                key=key, value=value
+            )
+
+    def _process(self, document_version):
+        raise NotImplementedError(
+            'Your %s class has not defined the required '
+            'process_document_version() method.' % self.__class__.__name__
+        )