Restructuring of from_data() and configuration

For better readability, correctness and maintainability the
body (which is still rather large, though) was restructured.

Regarding the signature, to be able to configure different fields in
the HTTP header the configuration parameter signature was added.

README.md

@@ -61,6 +61,7 @@ each hook has to be configured, It contains following fields:
   with `{{ /field/pointed/to }}`. Further `{{ event }}` and `{{
   signature }}` are valid variables as they contain the values from
   the regarding header fields of the http request.
+- signature: Name of the HTTP header field containing the signature.
 - secrets: List of secrets.
 - filters: List of filters.