Ability to alias "pluck_attributes" column names

This new feature adds the ability to alias the keys (column names) of
the results returned from "pluck_attributes" (while still maintaining
the proper type-casts).

* Add tests for "pluck_attributes" to cover existing functionality
* Add tests for "pluck_attributes" to cover new alias[ing] functionality
* Minor clean-up/refactoring
* Update README.md
Do you think we really need to use a hash with indifferent access here?