Use a schema validation library?

[geowurster]

Concerned about performance implications, so explain in readme how schema registration is not permanently supported and may change:

• [x] cerberus
• [x] Colander
• [x] schema - may not have long term support
• [x] jsonschema
• [x] voluptuous

[geowurster]

Chose voluptuous but it proved to be far too slow. Now we brew our own in gpsdio.validate. See #162.