Release 3.1.2

• Remove support for empty terms, as they must not be an empty string in 1.1 (1.0, too really).
• Add tests for keyword redefinition of @type has an empty map as a value.
• Allow aliases of @nest to be expanded term definitions, as well as simple.
• Missing script element should be LoadingDocumentFailed not InvalidScriptElement.
• Loading an HTML with no script element should be an error, not an empty document.
• Fix HTML libraries to deal with loading when both have been required by being explicit about which library to use.
• Use validate_scoped option to context and term creation.
• Fix bug in merge_node_map_graphs for @type.
• Update message text for "context overflow" exception.