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.

setup.cfg

@@ -1,5 +1,11 @@
 [tool:pytest]
-addopts = -v --cov pillar --cov-report term-missing --ignore node_modules -x
+addopts = -v --cov pillar --cov-report term-missing --ignore node_modules
+
+[mypy]
+python_version = 3.6
+warn_unused_ignores = True
+ignore_missing_imports = True
+follow_imports = skip
 
 [pep8]
 max-line-length = 100