Do not rely on JSON gem

[alsemyonov]

It makes it possible to use JSONAPI gem inside RubyMotion projects without having a lot of warnings on every run.

[beauby]

It makes sense to remove the JSON dependency, as it is there only for convenience, and causes issue with versioning (c.f. #34). In order to merge this, I would need appropriate modifications done there as well (basically removing the call to JSON.parse).

[beauby]

Fixes #28.

[beauby]

LGTM, just get rid of the call to JSON.parse in jsonapi/parse.rb.

[alsemyonov]

I consider that we should rather get back require 'json' for backward compatibility since JSON is in Ruby stdlib since earlier 2.*.*s or 1.9.*s, I believe. We just should not enforce users of the jsonapi gem to install or update json instead of using system one or specifically needed by other obstacles only because of they want to use JSON API parser gem.

Another way is to raise ArgumentError if we get any data except Hash to enforce JSONAPI gem users to parse JSON by themselves and give it to (sic!) JSONAPI.parse method? Why should they parse JSON before parsing JSONAPI?