Change history for XBlock SDK

These are notable changes in XBlock.

0.1 - In Progress

Make Scope enums (UserScope.* and BlockScope.*) into Sentinels, rather than just ints, so that they can have more meaningful string representations.
Rename export_xml to add_xml_to_node, to more accurately capture the semantics.
Allowed Runtime implementations to customize loading from block_types to XBlock classes.