Support The Workflow Status Category Resource

[jcouball]

This gem does not implement Workflow Status Category as a proper resource so it can be treated like an object.

This PR adds the JIRA::Resource::StatusCategory and relationship from JIRA::Resource::Status to this resource. This enables:

• Listing all status categores in Jira using client.StatusCategory.all
• Accessing the status category of a status via the status object. For example: status.status_category.id, status.status_category.name, etc. Before this change, the status category of a status was exposed as a hash via status.attrs[:statusCategory]

I believe this change is backward compatible even if client.Field.map_fields is called since the mapping would create status#statusCategory method which would not conflict with status#status_category. I also believe this gem would work if the server did not return statusCategory data in the status objects.

In addition to adding the status_category mocked responses, I have also added statusCategory data to the status mocked responses.

There is a surprising change needed to lib/jira/base.rb because the ids for statuscategory resources are an Integer where all other resources have String keys. This is how Jira returns these objects.

[jcouball]

@SimonMiaou this PR is ready for review.