Big refactoring of file storage handling

- Moved pillar.api.utils.{gcs,storage} to pillar.api.file_storage_backends
- Implemented GCS and local storage using abstract Bucket and Blob classes
- Removed file processing from the Blob class, and kept it in the
  file_storage/__init__.py class. That way storage and processing are
  kept separate.

requirements-dev.txt

@@ -8,6 +8,8 @@ responses==0.5.1
 pytest-cov==2.4.0
 mock==2.0.0
 
+# mypy-0.501 typed-ast-1.0.2
+
 # Secondary development requirements
 cookies==2.2.1
 coverage==4.3.4