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/paperless_tesseract/signals.py

@@ -0,0 +1,23 @@
+import re
+
+from .parsers import RasterisedDocumentParser
+
+
+class ConsumerDeclaration(object):
+
+    MATCHING_FILES = re.compile("^.*\.(pdf|jpg|gif|png|tiff|pnm|bmp)$")
+
+    @classmethod
+    def handle(cls, sender, **kwargs):
+        return cls.test
+
+    @classmethod
+    def test(cls, doc):
+
+        if cls.MATCHING_FILES.match(doc):
+            return {
+                "parser": RasterisedDocumentParser,
+                "weight": 0
+            }
+
+        return None