Include metadata in JSONParser output

[hamedahmadi]

The JSONParser will add metadata in the request under a top level key _meta.

This provides access to the data in the meta tag of the request.

[jerel]

@hamedahmadi this looks like a nice addition. What is the reason for choosing _meta as the key name rather than meta?

[hamedahmadi]

@jerel using _meta to prevent a clash in case the resource has a meta attribute.

[jerel]

@hamedahmadi ok, makes sense. Would you mind adding some simple documentation yet before I merge? Make a note about parsing incoming meta here: docs/usage.md#meta and add a section for JSONParser here with parse_metadata documented: docs/api.md

[hamedahmadi]

@jerel Thanks for the comment, updated.