Generate schemas from the Data Dictionary

This PR supercedes #7 and #8.

Instead of building schemas by hand (lame) we want a tool to generate them for us (cool). The script in this PR nets us fully DD@1.3 compliant schemas, with enumerations (as enums) and repeating elements (as subdocument arrays). Not included are Core fields and Core enumerations, since they aren't yet part of the released version of the spec. After this PR there will be a `1.4-wip` branch we can use to track new stuff.