Extract io layer to writer/reader class

Created by: stefanoborini

Extracted IO layer into writer and reader classes, and create model objects with support from the bundles.

force_bdss/io/tests/test_workflow_reader.py

+import json
+import unittest
+from six import StringIO
+
+from force_bdss.bundle_registry_plugin import BundleRegistryPlugin
+from force_bdss.io.workflow_reader import (
+    WorkflowReader,
+    InvalidVersionException, InvalidFileException)
+
+try:
+    import mock
+except ImportError:
+    from unittest import mock
+
+
+class TestWorkflowReader(unittest.TestCase):
+    def setUp(self):
+        self.mock_bundle_registry = mock.Mock(spec=BundleRegistryPlugin)
+        self.wfreader = WorkflowReader(self.mock_bundle_registry)
+
+    def test_initialization(self):
+        self.assertEqual(self.wfreader.bundle_registry,
+                         self.mock_bundle_registry)
+
+    def test_invalid_version(self):
+        data = {
+            "version": "2"
+        }
+
+        with self.assertRaises(InvalidVersionException):
+            self.wfreader.read(self._as_json_stringio(data))
+
+    def test_absent_version(self):
+        data = {
+        }
+
+        with self.assertRaises(InvalidFileException):
+            self.wfreader.read(self._as_json_stringio(data))
+
+    def _as_json_stringio(self, data):
+        fp = StringIO()
+        json.dump(data, fp)
+        fp.seek(0)
+
+        return fp
+
+    def