Use `go generate` to generate struct definitions automatically

[ChimeraCoder]

With Go 1.4, the go generate command was added, which allows us to tie in gojson more cleanly. (gojson is the same tool that was being used previously, just informally).

Automatically generating the struct definitions is more robust, cleaner, less error-prone, and also makes it easier for us to update the library if the API changes at any point in the future.