feat: refactor for pluggable consumers

I've broken out the OCR-specific code from the consumers and dumped it
all into its own app, `paperless_tesseract`.  This new app should serve
as a sample of how to create one's own consumer for different file
types.

Documentation for how to do this isn't ready yet, but for the impatient:

* Create a new app
    * containing a `parsers.py` for your parser modelled after
      `paperless_tesseract.parsers.RasterisedDocumentParser`
    * containing a `signals.py` with a handler moddelled after
      `paperless_tesseract.signals.ConsumerDeclaration`
    * connect the signal handler to
      `documents.signals.document_consumer_declaration` in
      `your_app.apps`
* Install the app into Paperless by declaring
  `PAPERLESS_INSTALLED_APPS=your_app`.  Additional apps should be
  separated with commas.
* Restart the consumer

src/documents/parsers.py

@@ -0,0 +1,45 @@
+import logging
+import shutil
+import tempfile
+
+from django.conf import settings
+
+
+class ParseError(Exception):
+    pass
+
+
+class DocumentParser(object):
+    """
+    Subclass this to make your own parser.  Have a look at
+    `paperless_tesseract.parsers` for inspiration.
+    """
+
+    SCRATCH = settings.SCRATCH_DIR
+
+    def __init__(self, path):
+        self.document_path = path
+        self.tempdir = tempfile.mkdtemp(prefix="paperless", dir=self.SCRATCH)
+        self.logger = logging.getLogger(__name__)
+        self.logging_group = None
+
+    def get_thumbnail(self):
+        """
+        Returns the path to a file we can use as a thumbnail for this document.
+        """
+        raise NotImplementedError()
+
+    def get_text(self):
+        """
+        Returns the text from the document and only the text.
+        """
+        raise NotImplementedError()
+
+    def log(self, level, message):
+        getattr(self.logger, level)(message, extra={
+            "group": self.logging_group
+        })
+
+    def cleanup(self):
+        self.log("debug", "Deleting directory {}".format(self.tempdir))
+        shutil.rmtree(self.tempdir)