Feature: add .find_by

Use `.find_by` to get first element only.
`.find_by` will call the `index`-Action and return the first resource.
Optionally passed params will be sent as `GET` parameters and can be used for filtering in the service' controller.
Fix specs for .find_by
Merge pull request #16 from nicolas-fricke/feature/find_by

Feature: add .find_by for retrieving first element only