libbalde/app: added balde_app_set_config_from_envvar

this function makes it easier to read configuration values from
environment variables.
libbalde/app: initialize user_data with NULL
libbalde/resources: adding cache for static resources. wip
libbalde/exceptions: added 3xx status codes
Merge branch 'master' into feature/cache-resources
libbalde/resources: etag cache done
libbalde/datetime: removed parse code. not needed at this point.
libbalde/resources: added tests for make_response_from_static_resource
capitalize status
Merge branch 'feature/cache-resources'
libbalde/httpd: added status code to logging
Merge branch 'master' into feature/doc-rework