Change the default config retrieval to parse data as JSON.
Add is_json to dummy config store.
Add opts which specifies which config keys are JSON.
Add test for opts.is_json.
Rename is_json to json.
Merge branch 'feature/issue-174-support-for-non-json-config-values' into develop
Merge branch 'develop' of github.com:praekelt/vumi-jssandbox-toolkit into develop