Allow further filtering of keys via :allowed_keys param

Will only filter from existing list of configured allowed keys
Bump version 0.0.6
Add spec license MIT

Fixes issue #3
Using `select` rather than additional array.

Missed it completely. Thanks @masylum.
Merge pull request #2 from teambox/feature/allowed-keys-as-params

Feature: Further filter keys using 'allowed_keys' param