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
Filter files for gem packaging.
Release 0.28.0
Feature: add .find_by! method for raising exception if no object can be loaded

`.find_by` will return `nil` if no object can be found and call optional callback on `nil`. Use `.find_by!` to raise an `Acfs::ResourceNotFound` exception if no object can be found. `.find_by!` will only invoke the optional callback if an object was successfully loaded.
Improve description for .find_by! method
Improve test for correct URL call when invoking .find_by

Use WebMock's #have_been_requested method on before defined request
Merge pull request #17 from nicolas-fricke/feature/find_by!

Feature: add .find_by! method, raises exception if no object can be loaded
Release 0.29.0