Initial engine framework

This commit adds the initial engine framework for Deckhand. Included
is the logic for parsing YAML files as well as validating them and
doing forward substitution as specified by the YAML file.

This commit also includes unit tests for the framework changes.

deckhand/errors.py

@@ -18,4 +18,4 @@ class ApiError(Exception):
 
 
 class InvalidFormat(ApiError):
-    pass
+    """The YAML file is incorrectly formatted and cannot be read."""