refactor api/pins

* use `current_member`
* use 1.9 style Hash syntax
* remove magic-number of ErrorCode
* use Member#pins association to find destroy pins
Merge branch 'refactor_api_pins' of https://github.com/onk/fastladder into onk-refactor_api_pins

Conflicts:
	spec/controllers/api/pin_controller_spec.rb