add additional features to the messaging schema validation

[hamilton]

The schema validation part of the Messaging is a great way to catch errors at the point of reporting from content scripts. It is, however, missing some features that would enable more sophisticated use in future studies:

• there is no way to mark a field as optional or alternatively specify which are required
• there is no way to specify more than one type

Some possibilities:

1. build these two features into the current schema validation. It wouldn't be too hard to take the syntax from json schema and implement those here.
2. use a well-tested / pre-existing schema validation library like ajv

[jonathanmayer]

use a well-tested / pre-existing schema validation library like ajv

I like this idea. We developed the current schema system as an ad hoc means of minimizing foreseeable bugs. Switching to a standard schema system and library should be (relatively) painless, since our schemas are simple and we can integrate a library into the build.