Remove unnecessary levels of sequences from the XML tree.

The old implementation created extra levels of sequences in the XML tree that were not needed. This didn't impact the generated xml, but it ended up getting reflected in the tests.
Add tests for core.

These tests are not comprehensive. In fact, they're mostly here to document how lists are converted into XML.
Bump version from 0.1.3-SNAPSHOT to 0.1.3.
Bump version from 0.1.3 to 0.1.3-SNAPSHOT.