RequestValidation objects always have attributes

When working with validation requests, there are always domains, thus there are
always zero or more validation request domains. This makes it easier to work
with the objects.

Based on xolphin#3

RequestValidation objects always have attributes

Release v1.5.0 (see CHANGELOG.md for updates)

Merge pull request #1 from ByteInternet/add_send_subscriber_agreement…

…_option

Add send_subscriber_agreement function